[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 precreate script, executed at instance creation time or when
the cube is added to an existing instance, before the schema is serialized.

This is typically to create groups referenced by the cube'schema.
"""