SMURF: Add a null INBEAM to mimic ACSIS.

Something has changed recently in SMURF.  GSD-converted data now
consistently fail in ORAC-DR reductions where they had previously
completed successfully.  At present MAKECUBE complains that there is
no INBEAM header present.  Adding such a header with a null value
seems to satisfy MAKECUBE's demand.  I note that early ACSIS data do
not have this header either, but MAKECUBE does not complain for those.