preferences: don't display variants checkbox if there is no variant
preferences: don't display variants checkbox if there is no variant

https://bugzilla.gnome.org/show_bug.cgi?id=660586

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