Change the use of == operator to isEmpty() in JsonObject (#2768)

Author: therepanic

gson/src/main/java/com/google/gson/JsonObject.java

@@ -164,7 +164,7 @@ public int size() {
    * @since 2.10.1
    */
   public boolean isEmpty() {
-    return members.size() == 0;
+    return members.isEmpty();
   }
 
   /**