lastgenre: Test empty last.fm result doesnt wipe

[JOJ0]

Add a test case that proves that issue 5991 is fixed by now.

Closes #5991 .

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.07%. Comparing base (3d3f5e3) to head (80376a9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6608   +/-   ##
=======================================
  Coverage   72.07%   72.07%           
=======================================
  Files         159      159           
  Lines       20633    20633           
  Branches     3273     3273           
=======================================
  Hits        14871    14871           
  Misses       5053     5053           
  Partials      709      709           

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Copilot]

Pull request overview

PR add regression test for lastgenre plugin so existing multi-value genres not get wiped when Last.fm give no tags (issue #5991). This fit beets plugin test suite, guard against old bug come back.

Changes:

• Add new parametrized case in test_get_genre for whitelist: True + keep_existing: True with empty Last.fm results.
• Assert "original fallback" keep both existing genres.