[dataimport] Add safety belt on "gen_rtype" in MetaGenerator, closes #3712892
Some cubes may well add to the META_RTYPES set, hence
crashing the gen_<somecubemeta> call.
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s specific hooks and operations"""