lastgenre: Apply canonicalization to genres of items with no Last.fm results

[Bergbok]

I've been configuring my canonicalization tree to remove tags I don't like, essentially using it like a blacklist, and noticed some tags not being removed after re-running the lastgenre command.

The reason for this is that when keep_existing is enabled and no Last.fm results are found, the existing genres are returned as-is without being processed through _resolve_genres(). This means existing genres that should be filtered by the canonicalization tree are kept unchanged.

As an example, if I had this in my canonicalization tree:

- '':
  - 'some tag'

and then ran lastgenre on a track with 'some tag', but which had no data on Last.fm, it wouldn't get removed from the track. This PR fixes that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.97%. Comparing base (83f1671) to head (67f7658).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6383      +/-   ##
==========================================
+ Coverage   68.96%   68.97%   +0.01%     
==========================================
  Files         140      140              
  Lines       18685    18683       -2     
  Branches     3056     3055       -1     
==========================================
+ Hits        12886    12887       +1     
+ Misses       5155     5154       -1     
+ Partials      644      642       -2     

Files with missing lines	Coverage Δ
beetsplug/lastgenre/__init__.py	71.00% <100.00%> (+0.89%)	⬆️

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[JOJ0]

I've been configuring my canonicalization tree to remove tags I don't like, essentially using it like a blacklist, and noticed some tags not being removed after re-running the lastgenre command.

A blacklist feature is in the pipeline but it will take a while until it's ready. 2 other lastgenre PR's need to go first. You could test from the branch but I suggest with a test library setup only for now: #5744