Enable nullability for the Xamarin.MacDev project.

[rolfbjarne]

Then disable it in every file, except a couple where any nullability issues are now fixed.

[Copilot]

Pull request overview

This PR enables nullable reference types for the Xamarin.MacDev project at the project level, then strategically disables it in most files while fixing nullability issues in two interface files.

Changes:

• Enabled <Nullable>enable</Nullable> in Xamarin.MacDev.csproj
• Added #nullable disable directives to 20 files with a consistent explanatory comment
• Fixed nullability issues in IAppleSdkVersion.cs and IAppleSdk.cs

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Xamarin.MacDev/Xamarin.MacDev.csproj	Enables nullable reference types at the project level
Xamarin.MacDev/IAppleSdkVersion.cs	Adds proper nullable annotations, conditional NotNullWhen attribute, and fixes null checking logic
Xamarin.MacDev/IAppleSdk.cs	Enables nullable reference types (no other changes needed)
Xamarin.MacDev/XamMacSdk.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/ProcessArgumentBuilder.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/PlatformAvailability.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/MonoTouchSdk.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/MobileProvisionIndex.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/MobileProvision.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/ManifestExtensions.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/MacOSXSdkVersion.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/MacOSXSdk.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/LoggingService.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/Keychain.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/IPhoneSdkVersion.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/IPhoneImageSizes.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/ExtendedVersion.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/EntitlementExtensions.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/AppleSdkVersion.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/AppleSdkSettings.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/AppleSdk.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/AppleIPhoneSdk.cs	Disables nullable reference types with explanatory comment
Xamarin.MacDev/AnalyticsService.cs	Disables nullable reference types with explanatory comment

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