Add default to site_template_base_dir context_processor

[ephes]

If there are no wagtail sites the site_template_bas_dir context_manager raises an DoesNotExist-Exception. This is bad if django-cast is used in one part of a site, but does not get used in another part. View tests for the non django-cast non wagtail part will run into this since they don't need a wagtail site.

Workaround: Make the context_processor pass even if there are no wagtail sites and set the base template dir name to a "does_not_exist" default.