webadmin: Allow empty gateway

I think the previous code was attempting to do the same thing, but did
so in an inconsistent manner.

Change-Id: I3aa805267e35c7f4f84b6c155db8bb002f5dc62a
Bug-Url: https://bugzilla.redhat.com/1075981
Signed-off-by: Lior Vernia <lvernia@redhat.com>