onions/detached and onions/current should return empty lists instead of errors

If you do a GETINFO query on onions/detached or onions/current and there aren't any, a 551 error is returned. Most other things just return an empty list/value in this case and I think these should too.

One consequence of this is that if you ask for onions/current with some other keys and there are no current onions, the whole query will fail.

So GETINFO onions/current should return onions/current= instead of an error.

For example you can try carml cmd getinfo orconn-status onions/current to see this behavior.