[Unity] Fix crash in ParsePush.jar related to empty push payload.

Fixes #104

Author: richardjrossiii

ParsePush/src/main/java/com/parse/ParsePushService.java

@@ -134,7 +134,7 @@ public IBinder onBind(Intent intent) {
   protected void onPushNotificationReceived(String pushPayloadString) {
     Log.i(TAG, "Push notification received. Payload: " + pushPayloadString);
     try {
-      if (!ParsePushUnityHelper.isApplicationPaused()) {
+      if (!ParsePushUnityHelper.isApplicationPaused() && pushPayloadString != null) {
         UnityPlayer.UnitySendMessage("ParseInitializeBehaviour",
             "OnPushNotificationReceived",
             pushPayloadString);