webadmin: AddRemoveRowWidget to only add ghost entry if no items

If there are items in the passed collection, then there's no need to
add a ghost entry by default, but only have a plus button next to the
last entry.

Change-Id: I120e9183d739213b3a26227cc8c3b738ee3b54dc
Bug-Url: https://bugzilla.redhat.com/1020861
Bug-Url: https://bugzilla.redhat.com/1058205
Signed-off-by: Lior Vernia <lvernia@redhat.com>