Avoid continuing with null message Ids

* Avoid NPE for messageId contains check

Author: Jeasmine

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

@@ -187,8 +187,10 @@ private void processInAppMessageJson(@NonNull JSONArray json) throws JSONExcepti
             for (int i = 0; i < json.length(); i++) {
                 JSONObject messageJson = json.getJSONObject(i);
                 OSInAppMessage message = new OSInAppMessage(messageJson);
-
-                newMessages.add(message);
+                // Avoid null checks later if IAM already comes with null id
+                if (message.messageId != null) {
+                    newMessages.add(message);
+                }
             }
 
             messages = newMessages;