MessageEvent: set user to None if not passed
MessageEvent: set user to None if not passed

Bots using CE now crash when someone stars a message because the user ID
is no longer provided:
http://meta.stackexchange.com/a/264580/229438

ByteCommander/ChatExchange6,ByteCommander/ChatExchange6,Charcoal-SE/ChatExchange,Charcoal-SE/ChatExchange,hichris1234/ChatExchange,hichris1234/ChatExchange