Fix creation of CG build without any RPM.
In case the module did not contain any RPM, _koji_rpms_in_tag still
tried to call the session.getRPMHeaders with an empty list. This results
in None being returned, which is something _koji_rpms_in_tag did
not count with and failed with traceback when trying to iterate
that None value.
In this commit, the _koji_rpms_in_tag returns an empty list early
if Koji tag does not contain any RPM. Therefore the
session.getRPMHeaders is not called at all in this case, because it
does not make any sense to get RPM Headers when Koji tag does not
contain any RPM.