Prior to this fix, if the last message of a thread had been made by a deleted user it would cause an exception as we would have no user to display, this commit uses a solution we have been using at other places: the null pattern, through the use of `Chat::NullUser.new`.
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.
