[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