TypeError: object of type 'NoneType' has no len()

[sentry]

Sentry Issue: BOT-1NS

TypeError: object of type 'NoneType' has no len()
(2 additional frame(s) were not displayed)
...
  File "bot/exts/moderation/infraction/_scheduler.py", line 85, in reapply_infraction
    expiry = dateutil.parser.isoparse(infraction["expires_at"]).replace(tzinfo=None)
  File "dateutil/parser/isoparser.py", line 37, in func
    return f(self, str_in, *args, **kwargs)
  File "dateutil/parser/isoparser.py", line 134, in isoparse
    components, pos = self._parse_isodate(dt_str)
  File "dateutil/parser/isoparser.py", line 208, in _parse_isodate
    return self._parse_isodate_common(dt_str)
  File "dateutil/parser/isoparser.py", line 213, in _parse_isodate_common
    len_str = len(dt_str)

Unhandled exception in on_member_join.

This seems to be caused when a user with a permanent mute leaves and rejoins the server.