Ignore null fields in model serialization

in Java clients.

Closes #1413

modules/swagger-codegen/src/main/resources/Java/libraries/okhttp-gson/JSON.mustache

@@ -23,10 +23,9 @@ public class JSON {
   public JSON(ApiClient apiClient) {
     this.apiClient = apiClient;
     gson = new GsonBuilder()
-      .serializeNulls()
       .registerTypeAdapter(Date.class, new DateAdapter(apiClient))
       .create();
-	}
+  }
 
   public Gson getGson() {
     return gson;