[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 schema"""