might help us figure out if https://github.com/vector-im/riot-web/issues/3868 has happened.
When a client retries a key upload, don't give an error if the signature has changed (but the key is the same). Fixes https://github.com/vector-im/riot-android/issues/1208, hopefully.
In the situation where all of a user's devices get deleted, we want to indicate this to a client, so we want to return an empty dictionary, rather than nothing at all.