Artist detail view is broken

[mgoltzsche]

Trying to access the artist detail view fails with the following error in case of some artists:

ERROR    2023-12-23 04:09:39,971 [1:Core-12 (_actor_loop)] mopidy.core.library
  BeetsBackend backend caused an exception.
Traceback (most recent call last):
  File "/usr/local/lib/python3.12/site-packages/mopidy/core/library.py", line 17, in _backend_error_handling
    yield
  File "/usr/local/lib/python3.12/site-packages/mopidy/core/library.py", line 230, in lookup
    result = future.get()
             ^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/pykka/_threading.py", line 68, in get
    raise exc_value
  File "/usr/local/lib/python3.12/site-packages/pykka/_actor.py", line 238, in _actor_loop_running
    response = self._handle_receive(envelope.message)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/pykka/_actor.py", line 349, in _handle_receive
    return callee(*message.args, **message.kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/mopidy_beets/library.py", line 169, in lookup
    tracks.sort(key=lambda t: (t.date, t.disc_no, t.track_no))
TypeError: '<' not supported between instances of 'int' and 'NoneType'

It looks like this happens when some tracks are missing some of the date, disc and track values that are supposed to be used as sort keys.