Solves the problem when posting and empty credential list e.g. in :

     http://localhost:8080/ws/1.1/credential.post?usertoken=e89787fbfbfc11f530f52b9e0d4cd0bc6d1ae676714f3799&credential_list=%7B%22credential_list%22%3A%5B%5D%7D&app_id=android-player-v1.0&format=json