[rql2sql] Test and fix some case of sql generation when an unstable variable (ie whose type change from one solution to another) is used in an EXISTS node but belongs to an outer scope. Closes #13518646.

# -*- 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"""