NavigationBarView: if there's no buttons lets revert to the default layout

I definitely had this in my source at some point but it must have been lost
in the shuffle.  If you don't want any keys on your navbar you might need a
bit of a slap... but regardless this prevents a potential NPE.

Change-Id: Ib6dc631d0a3fb80259d5bea11a47837a416d13ea