stacktrace on _reap_extra_connection()

Seen in rackd.log (https://oil-jenkins.canonical.com/artifacts/1c842f25-8a5d-4b4b-8e44-c8423571fe51/index.html)

2023-08-02 12:36:23 -: [critical] Unhandled Error
 Traceback (most recent call last):
   File "/usr/lib/python3.10/asyncio/base_events.py", line 600, in run_forever
     self._run_once()
   File "/usr/lib/python3.10/asyncio/base_events.py", line 1896, in _run_once
     handle._run()
   File "/usr/lib/python3.10/asyncio/events.py", line 80, in _run
     self._context.run(self._callback, *self._args)
   File "/snap/maas/29775/usr/lib/python3/dist-packages/twisted/internet/asyncioreactor.py", line 271, in _onTimer
     self.runUntilCurrent()
 --- <exception caught here> ---
   File "/snap/maas/29775/usr/lib/python3/dist-packages/twisted/internet/base.py", line 991, in runUntilCurrent
     call.func(*call.args, **call.kw)
   File "/snap/maas/29775/lib/python3.10/site-packages/provisioningserver/rpc/connectionpool.py", line 66, in _reap_extra_connection
     self.disconnect(conn)
   File "/snap/maas/29775/lib/python3.10/site-packages/provisioningserver/rpc/connectionpool.py", line 138, in disconnect
     return connection.transport.loseConnection()
 builtins.AttributeError: 'NoneType' object has no attribute 'loseConnection'