SitesConfigurationStep does not handle case of zero defined Sites

[swrichards]

The current step always runs Site.objects.get_current() but does not catch Site.DoesNotExist (if no sites) or ImproperlyConfigured (if no SITE_ID in settings, and not in a request context so the host can be used). This means that if the step is run on a database with no Sites, it will fail.

Reported by @SilviaAmAm and @sjoerdie .