[AIO] Connection real_nickname must not be set before registration

Some networks (IRCnet in particular) may reserve nicknames temporarily when you disconnect. The network will send `437 ERR_UNAVAILRESOURCE` during connection registration which means our real real nickname is not yet actually set.

Having it an empty string or `None` would help detecting this issue _during_ registration that our desired nickname is not available right now and the downstream implementation can decide what to do with this information, like use a temporary nickname and a timer to retry the real nickname later.

Without any subclassing, currently it is required to wait for 001 before believing `real_nickname` which will add unnecessary complexity.

Trace the connection-registration handling for ERR_UNAVAILRESOURCE (437), real_nickname, and the 001 welcome event. Verify the current state transitions and add or update tests so real_nickname remains empty or None until registration establishes the desired nickname, including the temporary-unavailable case.

Written by the indexing model from the issue text.