gnome: don't set XDG_CURRENT_DESKTOP if not available
gnome: don't set XDG_CURRENT_DESKTOP if not available

gnome-session assumes XDG_CURRENT_DESKTOP=GNOME if the env
variable isn't set, but not in the case that it's an empty
string.

dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton,dnschneid/crouton