[Java][OkHTTP] fix empty request body handling (#12172)

modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache

@@ -1356,7 +1356,7 @@ public class ApiClient {
                 reqBody = null;
             } else {
                 // use an empty request body (for POST, PUT and PATCH)
-                reqBody = RequestBody.create("", MediaType.parse(contentType));
+                reqBody = RequestBody.create("", contentType == null ? null : MediaType.parse(contentType));
             }
         } else {
             reqBody = serialize(body, contentType);

samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java

@@ -1151,7 +1151,7 @@ public class ApiClient {
                 reqBody = null;
             } else {
                 // use an empty request body (for POST, PUT and PATCH)
-                reqBody = RequestBody.create("", MediaType.parse(contentType));
+                reqBody = RequestBody.create("", contentType == null ? null : MediaType.parse(contentType));
             }
         } else {
             reqBody = serialize(body, contentType);

samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java

@@ -1229,7 +1229,7 @@ public class ApiClient {
                 reqBody = null;
             } else {
                 // use an empty request body (for POST, PUT and PATCH)
-                reqBody = RequestBody.create("", MediaType.parse(contentType));
+                reqBody = RequestBody.create("", contentType == null ? null : MediaType.parse(contentType));
             }
         } else {
             reqBody = serialize(body, contentType);

samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java

@@ -1230,7 +1230,7 @@ public class ApiClient {
                 reqBody = null;
             } else {
                 // use an empty request body (for POST, PUT and PATCH)
-                reqBody = RequestBody.create("", MediaType.parse(contentType));
+                reqBody = RequestBody.create("", contentType == null ? null : MediaType.parse(contentType));
             }
         } else {
             reqBody = serialize(body, contentType);

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java

@@ -1249,7 +1249,7 @@ public class ApiClient {
                 reqBody = null;
             } else {
                 // use an empty request body (for POST, PUT and PATCH)
-                reqBody = RequestBody.create("", MediaType.parse(contentType));
+                reqBody = RequestBody.create("", contentType == null ? null : MediaType.parse(contentType));
             }
         } else {
             reqBody = serialize(body, contentType);