[JavaScript] Avoid undefined headers with skipDefaultUserAgent enabled (#20792)

Prior to this commit, a JS client generated with skipDefaultUserAgent
enabled results in a src/ApiClient.js file that doesn't define
this.defaultHeaders. This breaks because this.defaultHeaders is later
directly passed to a superagent request which expects an object and not
undefined.

Fixes #20791.

modules/openapi-generator/src/main/resources/Javascript/libraries/javascript/ApiClient.mustache

@@ -56,7 +56,6 @@ class ApiClient {
         }
 
         <={{ }}=>
-	{{^skipDefaultUserAgent}}
 	{{#emitJSDoc}}
 	/**
          * The default HTTP headers to be included for all API calls.
@@ -65,10 +64,11 @@ class ApiClient {
          */
 	{{/emitJSDoc}}
         this.defaultHeaders = {
+            {{^skipDefaultUserAgent}}
             'User-Agent': '{{{httpUserAgent}}}{{^httpUserAgent}}OpenAPI-Generator/{{projectVersion}}/Javascript{{/httpUserAgent}}'
+            {{/skipDefaultUserAgent}}
         };
 
-	{{/skipDefaultUserAgent}}
         /**
          * The default HTTP timeout for all API calls.
          * @type {Number}

samples/client/petstore/javascript-es6/src/ApiClient.js

@@ -52,6 +52,14 @@ class ApiClient {
             'bearer_test': {type: 'bearer'}, // JWT
         }
 
+	/**
+         * The default HTTP headers to be included for all API calls.
+         * @type {Array.<String>}
+         * @default {}
+         */
+        this.defaultHeaders = {
+        };
+
         /**
          * The default HTTP timeout for all API calls.
          * @type {Number}