[msbuild] Fix nullability issue when compiling for legacy Xamarin.

[rolfbjarne]

And make sure we don't ignore any such warnings again for a few test projects.

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

• iOS (no change detected)
• tvOS (no change detected)
• watchOS (no change detected)
• macOS (no change detected)

NET (empty diffs)

• iOS: (empty diff detected)
• tvOS: (empty diff detected)
• MacCatalyst: (empty diff detected)
• macOS: (empty diff detected)

✅ API diff vs stable

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

Legacy Xamarin (stable) vs .NET

• iOS: vsdrops gist
• tvOS: vsdrops gist
• macOS: vsdrops gist

✅ Generator diff

Generator diff is empty

Pipeline on Agent
Hash: 7bacae2c814a006b2be5e5b44db118d6446f7445 [PR build]

[vs-mobiletools-engineering-service2]

💻 [PR Build] Tests on macOS M1 - Mac Big Sur (11.5) passed 💻

✅ All tests on macOS M1 - Mac Big Sur (11.5) passed.

Pipeline on Agent
Hash: 7bacae2c814a006b2be5e5b44db118d6446f7445 [PR build]

[vs-mobiletools-engineering-service2]

❌ [PR Build] Tests on macOS M1 - Mac Ventura (13.0) failed ❌

Failed tests are:

• xammac_tests

Pipeline on Agent
Hash: 7bacae2c814a006b2be5e5b44db118d6446f7445 [PR build]

[vs-mobiletools-engineering-service2]

📚 [PR Build] Artifacts 📚

Packages generated

View packages

• Microsoft.iOS.Bundle.16.2.254-ci.pr.gh17716.pkg
• Microsoft.iOS.Ref.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Runtime.ios-arm.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Runtime.ios-arm64.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Runtime.iossimulator-arm64.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Runtime.iossimulator-x64.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Runtime.iossimulator-x86.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Sdk.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Templates.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.iOS.Windows.Sdk.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.MacCatalyst.Bundle.16.2.254-ci.pr.gh17716.pkg
• Microsoft.MacCatalyst.Ref.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.MacCatalyst.Runtime.maccatalyst-arm64.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.MacCatalyst.Runtime.maccatalyst-x64.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.MacCatalyst.Sdk.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.MacCatalyst.Templates.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.macOS.Bundle.13.1.254-ci.pr.gh17716.pkg
• Microsoft.macOS.Ref.13.1.254-ci.pr.gh17716.nupkg
• Microsoft.macOS.Runtime.osx-arm64.13.1.254-ci.pr.gh17716.nupkg
• Microsoft.macOS.Runtime.osx-x64.13.1.254-ci.pr.gh17716.nupkg
• Microsoft.macOS.Sdk.13.1.254-ci.pr.gh17716.nupkg
• Microsoft.macOS.Templates.13.1.254-ci.pr.gh17716.nupkg
• Microsoft.NET.Sdk.iOS.Manifest-7.0.100.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.NET.Sdk.MacCatalyst.Manifest-7.0.100.16.2.254-ci.pr.gh17716.nupkg
• Microsoft.NET.Sdk.macOS.Manifest-7.0.100.13.1.254-ci.pr.gh17716.nupkg
• Microsoft.NET.Sdk.tvOS.Manifest-7.0.100.16.1.1042-ci.pr.gh17716.nupkg
• Microsoft.NET.Workload.iOS.16.2.254-ci.pr.gh17716.msi
• Microsoft.tvOS.Bundle.16.1.1042-ci.pr.gh17716.pkg
• Microsoft.tvOS.Ref.16.1.1042-ci.pr.gh17716.nupkg
• Microsoft.tvOS.Runtime.tvos-arm64.16.1.1042-ci.pr.gh17716.nupkg
• Microsoft.tvOS.Runtime.tvossimulator-arm64.16.1.1042-ci.pr.gh17716.nupkg
• Microsoft.tvOS.Runtime.tvossimulator-x64.16.1.1042-ci.pr.gh17716.nupkg
• Microsoft.tvOS.Sdk.16.1.1042-ci.pr.gh17716.nupkg
• Microsoft.tvOS.Templates.16.1.1042-ci.pr.gh17716.nupkg
• xamarin.ios-16.2.1.254.pkg
• xamarin.mac-9.1.1.254.pkg

Pipeline on Agent XAMBOT-1185.Monterey'
Hash: 7bacae2c814a006b2be5e5b44db118d6446f7445 [PR build]

[vs-mobiletools-engineering-service2]

🚀 [CI Build] Test results 🚀

Test results

✅ All tests passed on VSTS: simulator tests.

🎉 All 225 tests passed 🎉

Tests counts

✅ bcl: All 69 tests passed. Html Report (VSDrops) Download
✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests: All 1 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ fsharp: All 7 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ framework: All 8 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ generator: All 2 tests passed. Html Report (VSDrops) Download
✅ interdependent_binding_projects: All 7 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ install_source: All 1 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ introspection: All 8 tests passed. Html Report (VSDrops) Download
✅ linker: All 65 tests passed. Html Report (VSDrops) Download
✅ mac_binding_project: All 1 tests passed. Html Report (VSDrops) Download
✅ mmp: All 2 tests passed. Html Report (VSDrops) Download
✅ mononative: All 12 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ monotouch: All 25 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ mtouch: All 1 tests passed. Html Report (VSDrops) Download
✅ xammac: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 8 tests passed. Html Report (VSDrops) Download
✅ xtro: All 2 tests passed. [attempt 2] Html Report (VSDrops) Download

Pipeline on Agent
Hash: 7bacae2c814a006b2be5e5b44db118d6446f7445 [PR build]