Improve "URL only" field handling

Some Edge Delivery Services blocks only need an URL to work, like fragments block or video block. Currently we need 2 fields in UE for the URL itself and the link title as we need to render these as an `a` tag. This should be improved in the backend to make the link title optional and set the URL for link title if the it was empty.

The issue does not name a file or test. Start by locating the backend code that builds link fields for fragments and video blocks; verify that an empty link title uses the URL and that the title field can be omitted.

Written by the indexing model from the issue text.