[AudioToolbox] Fix nullability issue to fix broken build.

[rolfbjarne]

Fixes this compile error:

AudioToolbox/MusicTrack.cs(256,11): error CS8603: Possible null reference return.

[vs-mobiletools-engineering-service2]

❌ [PR Build] Tests failed on Build ❌

Tests failed on Build.

• Azure DevOps
• Html Report (VSDrops)

API diff

✅ API Diff from stable

View API diff

• iOS vsdrops gist
• macOS vsdrops gist
• tvOS vsdrops gist
• watchOS vsdrops gist
• macCatiOS vsdrops gist
• macCat vsdrops gist

View dotnet API diff

• dotnet-iOS vsdrops gist
• dotnet-macOS vsdrops gist
• dotnet-tvOS vsdrops gist
• dotnet-MacCatalyst vsdrops gist

View dotnet legacy API diff

• dotnet-legacy-iOS vsdrops gist
• dotnet-legacy-macOS vsdrops gist
• dotnet-legacy-tvOS vsdrops gist
• dotnet-legacy-MacCatalyst vsdrops gist

View dotnet iOS-MacCatalayst API diff

• dotnet-macCatiOS vsdrops gist

GitHub pages

Results can be found in the following github pages (it might take some time to publish):

• Test results
• API diff
• API & Generator diff

Test results

6 tests failed, 93 tests passed.

Failed tests

• monotouch-test/Mac [dotnet]/Debug [dotnet]: Failed (Test run failed.
  Tests run: 2539 Passed: 2413 Inconclusive: 10 Failed: 1 Ignored: 125)
• monotouch-test/Mac Catalyst [dotnet]/Debug [dotnet]: Failed (Tests run: 2704 Passed: 2497 Inconclusive: 35 Failed: 4 Ignored: 203)
• link sdk/Mac Catalyst [dotnet]/Debug [dotnet]: TimedOut (Execution timed out after 1200 seconds.
  No test log file was produced)
• xammac tests/Mac Modern/Debug: Failed (Test run failed.
  Tests run: 2544 Passed: 2423 Inconclusive: 10 Failed: 1 Ignored: 120)
• xammac tests/Mac Modern/Release: Failed (Test run failed.
  Tests run: 2541 Passed: 2419 Inconclusive: 10 Failed: 1 Ignored: 121)
• xammac tests/Mac Modern/Release (all optimizations): Failed (Test run failed.
  Tests run: 2541 Passed: 2418 Inconclusive: 10 Failed: 1 Ignored: 122)

Pipeline on Agent XAMBOT-1094.BigSur
Merge 652a353 into dc3e1e1

[rolfbjarne]

Test failures are unrelated:

• monotouch-test/Mac [dotnet]/Debug [dotnet]: https://github.com/xamarin/maccore/issues/2525
• monotouch-test/Mac Catalyst [dotnet]/Debug [dotnet]: https://github.com/xamarin/maccore/issues/2525
• link sdk/Mac Catalyst [dotnet]/Debug [dotnet]: https://github.com/xamarin/maccore/issues/2414
• xammac tests/Mac Modern/Debug: https://github.com/xamarin/maccore/issues/2525
• xammac tests/Mac Modern/Release: https://github.com/xamarin/maccore/issues/2525
• xammac tests/Mac Modern/Release (all optimizations): https://github.com/xamarin/maccore/issues/2525