webadmin: snapshot sub-tab - avoid redundant update

VmSnapshotListModel -> updateActionAvailability:
avoid update when getItems is null (to prevent an NPE).

Change-Id: If3469c20f2a4caceb96b40bbf542baacbadcf30e
Bug-Url: https://bugzilla.redhat.com/1058618
Signed-off-by: Daniel Erez <derez@redhat.com>