Fix NullReferenceException when push notification handler hasn't been registered.

When a push was received without a handler, we would previously crash. This doesn't exist as an issue on other platforms, as we use other notification delivery systems there.

Fixes #27.

Author: richardjrossiii

Parse/ParsePush.WinRT.cs

@@ -18,8 +18,11 @@ static ParsePush() {
         t.Result.PushNotificationReceived += (sender, args) => {
           PushNotificationReceived(ParseInstallation.CurrentInstallation, args);
 
-          var payload = PushJson(args);
-          parsePushNotificationReceived.Invoke(ParseInstallation.CurrentInstallation, new ParsePushNotificationEventArgs(payload));
+          var payload = PushJson(args);
+          var handler = parsePushNotificationReceived;
+          if (handler != null) {
+            handler.Invoke(ParseInstallation.CurrentInstallation, new ParsePushNotificationEventArgs(payload));
+          }
         }
       );
     }