main: Don't attempt to translate ""

During the refactoring done in commit  0d6c295, we ended up passing _("")
to g_option context_new(). This is a bad thing to do as "" is a magic
string for gettext. g_option_context_new() can handle NULL, so let's
use that here.

https://bugzilla.gnome.org/show_bug.cgi?id=707439