Fix generation of query parameters for optional/nullable function arguments (#3903)

* OpenAPITools/openapi-generator#3880 Fix generation of query parameters for optional/nullable function arguments

* OpenAPITools/openapi-generator#3880 Updated generated samples

modules/openapi-generator/src/main/resources/kotlin-client/api.mustache

@@ -29,7 +29,16 @@ import {{packageName}}.infrastructure.toMultiValue
     @Suppress("UNCHECKED_CAST"){{/returnType}}
     fun {{operationId}}({{#allParams}}{{paramName}}: {{{dataType}}}{{^required}}?{{/required}}{{#hasMore}}, {{/hasMore}}{{/allParams}}) : {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Unit{{/returnType}} {
         val localVariableBody: kotlin.Any? = {{#hasBodyParam}}{{#bodyParams}}{{paramName}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}null{{/hasFormParams}}{{#hasFormParams}}mapOf({{#formParams}}"{{{baseName}}}" to "${{paramName}}"{{#hasMore}}, {{/hasMore}}{{/formParams}}){{/hasFormParams}}{{/hasBodyParam}}
-        val localVariableQuery: MultiValueMap = {{^hasQueryParams}}mapOf(){{/hasQueryParams}}{{#hasQueryParams}}mapOf({{#queryParams}}"{{paramName}}" to {{#isContainer}}toMultiValue({{paramName}}.toList(), "{{collectionFormat}}"){{/isContainer}}{{^isContainer}}listOf("${{paramName}}"){{/isContainer}}{{#hasMore}}, {{/hasMore}}{{/queryParams}}){{/hasQueryParams}}
+        val localVariableQuery: MultiValueMap = {{^hasQueryParams}}mapOf()
+{{/hasQueryParams}}{{#hasQueryParams}}mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                {{#queryParams}}
+                {{^required}}if ({{paramName}} != null) {
+                    put("{{paramName}}", {{#isContainer}}toMultiValue({{paramName}}.toList(), "{{collectionFormat}}"){{/isContainer}}{{^isContainer}}listOf({{paramName}}.toString()){{/isContainer}})
+                }{{/required}}{{#required}}put("{{paramName}}", {{#isContainer}}toMultiValue({{paramName}}.toList(), "{{collectionFormat}}"){{/isContainer}}{{^isContainer}}listOf({{paramName}}.toString()){{/isContainer}}){{/required}}
+                {{/queryParams}}
+            }
+            {{/hasQueryParams}}
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf({{#hasFormParams}}"Content-Type" to {{^consumes}}"multipart/form-data"{{/consumes}}{{#consumes.0}}"{{MediaType}}"{{/consumes.0}}{{/hasFormParams}}{{^hasHeaderParams}}){{/hasHeaderParams}}{{#hasHeaderParams}}{{#hasFormParams}}, {{/hasFormParams}}{{#headerParams}}"{{baseName}}" to {{#isContainer}}{{paramName}}.joinToString(separator = collectionDelimiter("{{collectionFormat}}")){{/isContainer}}{{^isContainer}}{{paramName}}.toString(){{/isContainer}}{{#hasMore}}, {{/hasMore}}{{/headerParams}}){{/hasHeaderParams}}
         val localVariableConfig = RequestConfig(
             RequestMethod.{{httpMethod}},

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -98,7 +98,10 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     fun findPetsByStatus(status: kotlin.Array<kotlin.String>) : kotlin.Array<Pet> {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("status" to toMultiValue(status.toList(), "csv"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("status", toMultiValue(status.toList(), "csv"))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,
@@ -129,7 +132,10 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     fun findPetsByTags(tags: kotlin.Array<kotlin.String>) : kotlin.Array<Pet> {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("tags" to toMultiValue(tags.toList(), "csv"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("tags", toMultiValue(tags.toList(), "csv"))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -188,7 +188,11 @@ class UserApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCl
     @Suppress("UNCHECKED_CAST")
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("username" to listOf("$username"), "password" to listOf("$password"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("username", listOf(username.toString()))
+                put("password", listOf(password.toString()))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -98,7 +98,10 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     fun findPetsByStatus(status: kotlin.Array<kotlin.String>) : kotlin.Array<Pet> {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("status" to toMultiValue(status.toList(), "csv"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("status", toMultiValue(status.toList(), "csv"))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,
@@ -129,7 +132,10 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     fun findPetsByTags(tags: kotlin.Array<kotlin.String>) : kotlin.Array<Pet> {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("tags" to toMultiValue(tags.toList(), "csv"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("tags", toMultiValue(tags.toList(), "csv"))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -188,7 +188,11 @@ class UserApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCl
     @Suppress("UNCHECKED_CAST")
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("username" to listOf("$username"), "password" to listOf("$password"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("username", listOf(username.toString()))
+                put("password", listOf(password.toString()))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -98,7 +98,10 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     fun findPetsByStatus(status: kotlin.Array<kotlin.String>) : kotlin.Array<Pet> {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("status" to toMultiValue(status.toList(), "csv"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("status", toMultiValue(status.toList(), "csv"))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,
@@ -129,7 +132,10 @@ class PetApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCli
     @Suppress("UNCHECKED_CAST")
     fun findPetsByTags(tags: kotlin.Array<kotlin.String>) : kotlin.Array<Pet> {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("tags" to toMultiValue(tags.toList(), "csv"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("tags", toMultiValue(tags.toList(), "csv"))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -188,7 +188,11 @@ class UserApi(basePath: kotlin.String = "http://petstore.swagger.io/v2") : ApiCl
     @Suppress("UNCHECKED_CAST")
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableBody: kotlin.Any? = null
-        val localVariableQuery: MultiValueMap = mapOf("username" to listOf("$username"), "password" to listOf("$password"))
+        val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
+            .apply {
+                put("username", listOf(username.toString()))
+                put("password", listOf(password.toString()))
+            }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         val localVariableConfig = RequestConfig(
             RequestMethod.GET,