db116ae Fix creation of CG build without any RPM.

Authored and Committed by jkaluza 5 years ago
    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.