gh-79033: Try to fix asyncio.Server.wait_closed() again (GH-111336)

* Try to fix asyncio.Server.wait_closed() again

I identified the condition that `wait_closed()` is intended
to wait for: the server is closed *and* there are no more
active connections.

When this condition first becomes true, `_wakeup()` is called
(either from `close()` or from `_detach()`) and it sets `_waiters`
to `None`. So we just check for `self._waiters is None`; if it's
not `None`, we know we have to wait, and do so.

A problem was that the new test introduced in 3.12 explicitly
tested that `wait_closed()` returns immediately when the server
is *not* closed but there are currently no active connections.
This was a mistake (probably a misunderstanding of the intended
semantics). I've fixed the test, and added a separate test that
checks exactly for this scenario.

I also fixed an oddity where in `_wakeup()` the result of the
waiter was set to the waiter itself. This result is not used
anywhere and I changed this to `None`, to avoid a GC cycle.

* Update Lib/asyncio/base_events.py

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Co-authored-by: Carol Willing <carolcode@willingconsulting.com>

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