webadmin: prevent npe on VdcActionUtils canExecute

VmSnapshotListModel -> updateActionAvailability:
passing an empty list to VdcActionUtils.canExecute
when VM is null to avoid an NPE.

Change-Id: I63e3570b48f8039642a5f1ce3af1f9c4599d688c
Signed-off-by: Daniel Erez <derez@redhat.com>