[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
[MESSAGES CONTROL]
disable-msg = C0301
[VARIABLES]
# Enable / disable this checker
enable-variables = yes
additional-builtins = _, display_name
[BASIC]
required-attributes=
attr-rgx = (w|[a-z_][a-z0-9_]{2,30})
variable-rgx = (w|[a-z_][a-z0-9_]{2,30})$
argument-rgx = (w|[a-z_][a-z0-9_]{2,30})$
[TYPECHECK]
generated-members=debug,info,notice,warning,error,critical,exception