Fixed nullable reference type (NRT) annotations in the MSBuild Tasks to follow repository guidelines as specified in copilot-instructions.md.

Changes Made

During investigation, found that most MSBuild task files in src/Xamarin.Android.Build.Tasks/Tasks/ already have proper #nullable enable directives and correct property annotations. Only one file needed updates:

Fixed Legacy/ResolveAndroidTooling.cs:

• ✅ Changed #nullable disable to #nullable enable
• ✅ Updated [Output] property to be nullable: public string? TargetFrameworkVersion { get; set; }
• ✅ Converted string.IsNullOrWhiteSpace() calls to use extension method pattern: property.IsNullOrWhiteSpace()
• ✅ Added proper null safety checks for method parameters and return values
• ✅ Fixed all nullable reference warnings and compilation errors

Verification

• ✅ All MSBuild task files now have #nullable enable directive
• ✅ All properties follow the correct patterns:
  • Non-required: public string? PropertyName { get; set; }
  • Required: [Required] public string PropertyName { get; set; } = "";
  • Output: [Output] public string? PropertyName { get; set; }
• ✅ Build succeeds without errors
• ✅ Changes are minimal and surgical (22 insertions, 11 deletions in 1 file)

The repository's MSBuild Tasks are now fully compliant with the nullable reference types guidelines.

Fixes #10277.

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