Use None as default values for multi-valued fields in hooks.Info

[snejus]

Fix empty list treated as non-empty field value in _apply_metadata

Fixes #6403

Thanks to @aereaux for reporting and helpful debugging!

Problem

In beets/autotag/__init__.py, _apply_metadata skipped overwriting existing field values with None to avoid clearing data unintentionally. However, an empty list ([]) was not treated the same way — it would still overwrite existing field values, effectively clearing multi-valued fields (e.g. genres) when the tag source returned nothing.

Fix

Keep multi-value field values as None by default in beets.autotag.hooks.Info subclasses:

def __init__(
    ...
    genres: list[str] | None = None,
    ...
):
    ...
    # Before
    self.genres = genres or []
    # After
    self.genres = genres

Test changes

• Added genres=["Rock", "Pop"] to the test album fixture to expose the bug: album genres were not being propagated to tracks due to the empty-list issue, since empty track-level genres overwrote them.
• Removed the @pytest.mark.xfail marker once the fix made the test pass.
• Consolidated ~20 granular ApplyTest methods and the separate ApplyCompilationTest class into a single data-driven test_autotag_items test, reducing noise and improving coverage clarity.
• Moved autotag tests into test/autotag/ to better reflect module structure.

[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.

[snejus]

Note I cherry-picked the test refactor commit from #6165.

[Copilot]

Pull request overview

PR make autotag metadata apply stop clearing multi-value fields when tag source give empty list (like genres). This fit core autotag flow so mbsync/tag update not delete existing list data by accident.

Changes:

• Treat [] same as None in _apply_metadata unless field allowed in overwrite_null
• Move autotag tests into test/autotag/ and collapse many small apply tests into one data-driven test
• Update fixture to include genres=["Rock","Pop"] so empty-list bug show up in tests

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
beets/autotag/init.py	Skip applying empty lists so existing multi-value fields not wiped
test/test_autotag.py	Remove old autotag test module (relocated)
test/autotag/test_autotag.py	New autotag test module; adds genres regression coverage and consolidates apply tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.33%. Comparing base (86fac4b) to head (a8b8aa9).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6405   +/-   ##
=======================================
  Coverage   69.33%   69.33%           
=======================================
  Files         141      141           
  Lines       18793    18793           
  Branches     3061     3061           
=======================================
  Hits        13031    13031           
  Misses       5117     5117           
  Partials      645      645           

Files with missing lines	Coverage Δ
beets/autotag/hooks.py	99.21% <100.00%> (ø)

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[aereaux]

This looks like it fixes #6403 for me, thanks!