webadmin: TypeAheadListBox to not override acceptable null

Altered ListModelTypeAheadListBox to only visually override null
values if they're not allowed, so that when clicking outside of the
list box it won't seem like another choice was chosen (when in reality
the backing value is still null).

Change-Id: I35b34097c9068f2baaa0573c74ec6c930b915f71
Bug-Url: https://bugzilla.redhat.com/1042872
Signed-off-by: Lior Vernia <lvernia@redhat.com>