LUQueryInstances: fix querying for nic data

Currently we support querying for "mac" "ip" or "bridge", meaning "the
one of the first nic. We are not checking that there is a first nic,
though, and thus could incur in errors. This patch fixes it by returning
"None" should there be no such nic, as it's done when explicitely asking
for a nic via nic.<field>/<N>

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>