fix(gen:app:socket): use `''` instead `null` as URL to open ioSocket

With socket.io 1.0.6, when opening ioSocket with null as URL, io() will
omit the option object. As a result, `query` and Auth Token will not be
sent through handshake.
Merge pull request #435 from gregwym/master

fix(gen:app:socket): use `''` instead `null` as URL to open ioSocket