restapi: Do not attempt to extend a block domain if LUNs data not supplied

When editing a block domain via REST, in case the storage data was not
supplied there is still a comparison between the old an new LUNs which
results in an NPE, in that case we just skip the comparison if the data
was not sent in the request

Change-Id: Ibf4b28c2dbc289b88107f1e7fdc19127fb536326
Bug-Url: https://bugzilla.redhat.com/1115966
Signed-off-by: Tal Nisan <tnisan@redhat.com>