This PR systematically modernizes MSBuild task properties in src/Xamarin.Android.Build.Tasks/Tasks/ to follow the repository's nullable reference types guidelines as outlined in .github/copilot-instructions.md.

Changes Made

Updated 4 MSBuild task files from #nullable disable to #nullable enable with proper nullable annotations:

• Javac.cs: Updated properties and converted string.IsNullOrEmpty() calls to extension method pattern
• Aot.cs: Updated properties with proper null handling for nullable property access
• GetJavaPlatformJar.cs: Updated properties, method signatures, and comprehensive null handling
• CheckGoogleSdkRequirements.cs: Updated properties and nullable value type handling

Pattern Updates Applied

Required Properties: Added default values

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

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

Non-Required Properties: Made nullable

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

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

Output Properties: Made nullable since they are set during execution

// Before
[Output]
public string OutputProperty { get; set; }

// After
[Output] 
public string? OutputProperty { get; set; }

String Checks: Converted to extension method pattern

// Before
if (!string.IsNullOrEmpty(property)) { }

// After
if (!property.IsNullOrEmpty()) { }

Scope

This addresses part of the broader modernization effort across 145 task files:

• ✅ 4 files converted from #nullable disable to #nullable enable
• 🔄 20 remaining files with #nullable disable to convert
• 🔄 117 files without nullable pragmas need evaluation

Testing

All updated files compile successfully without nullable reference type warnings or errors. The changes preserve existing functionality while improving type safety.

Addressing #10277.

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