fix the issue 890 which throw exception Cannot destructure property 'message' of 'tracker.fetch(...)' as it is null

lib/client/message-tracker/index.js

@@ -104,7 +104,7 @@ module.exports = function messageTrackerFactory (options) {
       return { message: abandonedMsg, callback: abandonedMsg.cb }
     }
 
-    return null
+    return { message: null, callback: null }
   }
 
   /**