fix(AudioTag): null-safe Diff() for OriginalReleaseDate

[csmarshall]

Summary

AudioTag.Diff() throws System.InvalidOperationException: Nullable object must have a value when the file's OriginalReleaseDate is set but the metadata being compared against has it null (or vice versa). Three-line fix: only apply the year-only shortcut when both sides have a value.

Context

Hit consistently after migrating an existing Readarr/Goodreads install to the hardcover image — Hardcover metadata commonly returns null release dates for works where Goodreads filled them in. Stack trace from a live failure:

[v0.4.20.129] System.InvalidOperationException: Nullable object must have a value.
   at NzbDrone.Core.MediaFiles.AudioTag.Diff(AudioTag other) in ./Readarr.Core/MediaFiles/AudioTag.cs:line 493
   at NzbDrone.Core.MediaFiles.AudioTagService.WriteTags(BookFile trackfile, Boolean newDownload, Boolean force) in ./Readarr.Core/MediaFiles/AudioTagService.cs:line 162
   at NzbDrone.Core.MediaFiles.BookImport.ImportApprovedBooks.Import(List`1 decisions, ...) in ./Readarr.Core/MediaFiles/BookImport/ImportApprovedBooks.cs:line 253

Downstream effect: ImportApprovedBooks bails on the file's import when the tag-write step throws, so no BookFile row gets written. In my install I saw the import scan run with writeaudiotags=Sync log a handful of explicit Couldn't import book warnings citing this stack trace, and ended with BookFiles = 0 after metadata refresh + identification had completed for ~14k files. Setting writeaudiotags=No is the workaround that unblocked the import (BookFiles populated to ~13.9k cleanly); this PR is the proper fix.

Root cause

The outer ?.Date != other.?.Date check correctly passes when one side is null. The inner ID3v2.3 year-only shortcut then dereferences other.OriginalReleaseDate.Value.Year unconditionally, crashing when other is null.

The companion Date diff a few lines above is already null-safe (uses HasValue before .Value); this branch is just an oversight.

Fix

                 // Id3v2.3 tags can only store the year, not the full date
                 if (OriginalReleaseDate.HasValue &&
                     OriginalReleaseDate.Value.Month == 1 &&
-                    OriginalReleaseDate.Value.Day == 1)
+                    OriginalReleaseDate.Value.Day == 1 &&
+                    other.OriginalReleaseDate.HasValue)
                 {

When other is null, fall through to the existing else branch which already handles HasValue on either side and emits a clean diff entry.

Verification

• Crash signature reproduced in my install on the current hardcover-v0.4.20.129 image (logs above).
• Code review: the fix matches the null-safety pattern already in use for the sibling Date diff a few lines above.
• Built locally: dotnet build NzbDrone.Core/Readarr.Core.csproj -c Release clean — 0 warnings, 0 errors.
• CI to verify no regressions in the existing test suite.

No existing unit test covers AudioTag.Diff() in this repo (checked under src/ for any *AudioTag*Test* files — none). Glad to add one if wanted; didn't want to scope-creep without a signal.