WebSocket Impl code style

services/mogo-service-api/src/main/java/com/mogo/service/connection/IMogoOnWebSocketMessageListener.java

@@ -5,9 +5,13 @@ package com.mogo.service.connection;
  */
 public interface IMogoOnWebSocketMessageListener< T > {
 
-    WebSocketMsgType getDownLinkType();
+    default WebSocketMsgType getDownLinkType(){
+        return null;
+    }
 
-    WebSocketMsgType getUpLinkType();
+    default WebSocketMsgType getUpLinkType(){
+        return null;
+    }
 
     default void onMsgReceived(T obj){