Let some code support "media without slug"

Let some code support "media without slug"

The upcoming SQL database will allow having media without a
slug again. This might especially be useful for API
support, where the uploaded image (media) does not have
*any* information to generate a slug from.

This code change mostly allows the sql backend to have no
slug in its model and improves some parts to properly
handle that.

It specifically does not have any web frontend support to
edit/create MediaEntries with no slug.