widget-irc: don't check 'username' using a regexp
widget-irc: don't check 'username' using a regexp

It's an optional parameter so can be not set.

https://bugzilla.gnome.org/show_bug.cgi?id=678307

Distrotech/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets