Authz service fails to deserialize room deletion events in some cases

The UUID-related changes refactored RoomDeletedEvent so its properties id and ownerId are no longer optional. In some cases the ownerId is not set in the event which now causes deserialization errors. Because the ownerId is not used by the authz service, it should be possible to remove it from the model.

The issue cannot be reproduced by manually deleting rooms (neither directly nor by deleting the user) in which case the ownerId is sent with the event. It should only occur when room deletion is automatically scheduled (e.g. demo rooms). In the latter case, the ownerId is omitted.

Regression-MR: !647 (merged)