Crash reported by several people pointing null from getTags success (#934)

* Added null and empty check for `getTags` response and assign to empty json string if necessary

Author: mikechoch

OneSignalSDK/onesignal/src/main/java/com/onesignal/OneSignalStateSynchronizer.java

@@ -27,7 +27,6 @@
 
 package com.onesignal;
 
-import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 
 import org.json.JSONException;

OneSignalSDK/onesignal/src/main/java/com/onesignal/UserStatePushSynchronizer.java

@@ -27,6 +27,13 @@ GetTagsResult getTags(boolean fromServer) {
                 @Override
                 void onSuccess(String responseStr) {
                     serverSuccess = true;
+
+                    // This should not typically come from the server as null or empty, but due to Issue #904
+                    // This check is added and will prevent further crashes
+                    // https://github.com/OneSignal/OneSignal-Android-SDK/issues/904
+                    if (responseStr == null || responseStr.isEmpty())
+                        responseStr = "{}";
+
                     try {
                         JSONObject lastGetTagsResponse = new JSONObject(responseStr);
                         if (lastGetTagsResponse.has("tags")) {