Fix Android No Tags Null Pointer Exception

• In cases where a user had no tags and the app called getTags(), the serializer wrapper would attempt to iterate through keys on a null object causing a null pointer exception
• Fixed issue by adding an early return in the serializer

Author: Nightsd01

android/src/main/java/com/onesignal/flutter/OneSignalSerializer.java

@@ -199,6 +199,9 @@ static private HashMap<String, Object> convertAndroidBackgroundImageLayoutToMap(
     static public HashMap<String, Object> convertJSONObjectToHashMap(JSONObject object) throws JSONException {
         HashMap<String, Object> hash = new HashMap<>();
 
+        if (object == null)
+           return hash;
+
         Iterator<String> keys = object.keys();
 
         while (keys.hasNext()) {