This PR systematically updates MSBuild task classes in src/Xamarin.Android.Build.Tasks/Tasks/ to follow the repository's nullable reference types guidelines from copilot-instructions.md.

Changes Made

Successfully converted 5 MSBuild task files from #nullable disable to #nullable enable with proper nullable patterns:

Files Updated:

• ✅ Lint.cs - Android Lint tool task
• ✅ GenerateLibraryResources.cs - Library R.java generation task
• ✅ GenerateJniRemappingNativeCode.cs - JNI remapping native code generation
• ✅ ValidateJavaVersion.cs - Java version validation task
• ✅ GenerateResourceDesignerIntermediateClass.cs - Resource designer class generation

Property Pattern Updates Applied:

Before:

#nullable disable

[Required]
public string RequiredProperty { get; set; }

public string NonRequiredProperty { get; set; }
public ITaskItem[] NonRequiredItemGroup { get; set; }

After:

#nullable enable

[Required]
public string RequiredProperty { get; set; } = "";

public string? NonRequiredProperty { get; set; }
public ITaskItem[]? NonRequiredItemGroup { get; set; }

Additional Improvements:

• Converted string.IsNullOrEmpty() calls to extension method: property.IsNullOrEmpty()
• Added null-coalescing operators for safe nullable handling: property ?? ""
• Used null-forgiving operators where null checks exist: property!.Member
• Updated constructor patterns to use new array syntax: = [] instead of Array.Empty<T>()
• Fixed null checks in foreach loops and method parameters

Testing

• ✅ All changes build successfully with .NET 10
• ✅ No breaking changes to existing functionality
• ✅ Follows repository coding standards and nullable patterns

Remaining Work

This is part of a larger effort to modernize all MSBuild tasks:

• 9 remaining files with #nullable disable need conversion
• 90 additional files without nullable directives need #nullable enable added

The patterns established in this PR provide a template for completing the remaining modernization work.

Addressing #10277.

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