[webconfig] fallback to default configs if https ones are not defined When cubicweb serves a request with an https scheme, it tries to access ``https_uiprops`` and ``https_datadir_urls``. However if those are not defined we should fallback to unsecure url instead of crashing with a traceback. There are very valid usecases for undefined https url. For example if your ``base_url``, ``datadir_url`` and uiprops content are already https urls. Closes #2786643

# -*- 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 entity's classes"""