Handle listen_task = None while reconnecting

[NicolaiSoeborg]

Got this error, not sure why, but I think it should be safe to skip the await and continue the reconnect method if self.listen_task is None.

[ERROR@mau.periodic_reconnect] Error while reconnecting @nick:homeserver
Traceback (most recent call last):
  File "/path/mautrix-facebook/mautrix_facebook/__main__.py", line 159, in _periodic_reconnect_loop
    await user.reconnect()
  File "/path/mautrix-facebook/mautrix_facebook/user.py", line 445, in reconnect
    await self.listen_task
 TypeError: object NoneType can't be used in 'await' expression

(I'm running on newest master, i.e. 7562745)

[tulir]

Not entirely sure if this is safe, I need to make sure there's something else blocking reconnection attempts if the user got logged out