[java-jersey2] Fix empty body when form parameters supplied (#5169)

Author: ind1go

libraries/jersey2/ApiClient.mustache

@@ -573,7 +573,7 @@ public class ApiClient {
       entity = Entity.entity(form, MediaType.APPLICATION_FORM_URLENCODED_TYPE);
     } else {
       // We let jersey handle the serialization
-      entity = Entity.entity(obj, contentType);
+      entity = Entity.entity(obj == null ? Entity.text("") : obj, contentType);
     }
     return entity;
   }
@@ -732,7 +732,7 @@ public class ApiClient {
       }
     }
 
-    Entity<?> entity = (body == null) ? Entity.json("") : serialize(body, formParams, contentType);
+    Entity<?> entity = serialize(body, formParams, contentType);
 
     Response response = null;