restapi: Null check on domain volume group when attempting to extend domain

When editing a block domain via REST, the LUN passed in the new entity can
come either from the LUNs collection and in case it's empty from the volume
group property and it's LUNs, added a null check on the volume group since
when you do not provide LUNs it automatically falls back to volume group
which might be null

Change-Id: Ic762bdfb4c01d7b9344c94878d6662b9866a4090
Bug-Url: https://bugzilla.redhat.com/1115966
Signed-off-by: Tal Nisan <tnisan@redhat.com>