update NIC: having no custom properies is valid

Older Engines, that preceded per-vNIC custom properties (Vdsm commit
38b1f8fb) do not send the "custom" element at all. This has to be
interpreted exactly as an empty custom element.

Bug-Url: https://bugzilla.redhat.com/1047646
Change-Id: If510f012039304ad16000d46d9d72a3e035539f5
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22922
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Assaf Muller <amuller@redhat.com>