coreartist: Fix None in year sort
The album sorter failed if the year was not available for an album.
Add a condition for this case.
Error in case:
TypeError: '>' not supported between instances of 'NoneType' and 'NoneType'
Traceback (most recent call last):
File "/usr/lib/python3.11/site-packages/gi/overrides/__init__.py", line 351, in wrap
return func(a, b, *user_data)
^^^^^^^^^^^^^^^^^^^^^^
File ".../coreartist.py", line 75, in _album_sort
if year_a > year_b: