e.g. in https://musicbrainz.org/ws/2/release-group/2d340d43-a128-42c9-8bba-c92f26fc41be?fmt=json&inc=artist-rels+url-rels

The target entity objects there have "relations": [] outputted, but that doesn't mean there are no relations, it's just that they're not even loaded (or intended to be).

For top-level entities that /actually/ have no relationships, we output the same thing, and it means something different.