This would cause an error when deleting the original message of a thread, due to the non existing `last_message`. This fix is implemented using the null pattern.
Note this commit is also using this opportunity to unify naming of null objects, `Chat::DeletedUser` becomes `Chat::NullUser`, it feels better to have a name describing what is the object, instead of a name describing why this object has to be used, which can change depending on cases.
