fix a bug where TileJson request ran callbacks twice (second time with null object)

Author: brnkhy

Runtime/Mapbox/BaseModule/Data/Platform/TileJSON/TileJSON.cs

@@ -44,8 +44,10 @@ public IAsyncRequest Get(string tilesetName, Action<TileJSONResponse> callback)
                         }
                         callback(tileJSONResponse);
                     }
-
-                    callback(null);
+                    else
+                    {
+                        callback(null);    
+                    }
                 }
                 , _timeout
             );