Following I52be5f1ee59c242db6ed0efd2ecfdc380a565171 in mediawiki, some lower-level services in Mediawiki expect to only be given special value false when they should deal with wiki's local database, rather than the string representing the database name.

As noticed in T406777, this expectation is currently not fulfilled by Wikibase's DatabaseEntitySource class.

The database name that this class exposes through DatabaseEntitySource::getDatabaseName is meant to be used as input to services dealing with Mediawiki database. As such, that method should likely be using the special false value in applicable scenarios rather then returning string name, to honour expectations in Medawiki