fix string comparison structs crash when compared with default values

[xj-ms]

The string comparison structs are meant to be used as column data types. When their default values are used, the internal string will be null, which triggers a null reference exception in their comparison logic or ToString.

This PR fixes this issue, adds null annotations and unit tests

[helenkzhang]

Approved with suggestions

[Copilot]

Pull request overview

This PR fixes null reference exceptions that occur when string comparison structs (StringWithLogicalComparison and StringWithCaseInsensitiveComparison) are used with their default values. These structs serve as column data types in the SDK, and when instantiated via default values (without calling the constructor), their internal string field is null, causing crashes in comparison and ToString operations.

Key Changes:

• Added nullable annotations (#nullable enable) and marked internal string fields as nullable to properly handle default struct values
• Updated ToString() methods to return empty strings when the internal string is null
• Added null pointer checks in the comparison logic to prevent crashes when comparing default values
• Added comprehensive unit tests for both structs covering default value scenarios

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/Microsoft.Performance.SDK/StringWithLogicalComparison.cs	Enabled nullable reference types, made source field nullable, and updated ToString methods to handle null values
src/Microsoft.Performance.SDK/StringWithCaseInsensitiveComparison.cs	Enabled nullable reference types, made source field nullable, and updated ToString methods to handle null values
src/Microsoft.Performance.SDK/CompareNumericWithExceptionsMethods.cs	Added null pointer checks at the start of comparison method to safely handle null strings from default struct values
src/Microsoft.Performance.SDK.Tests/StringWithLogicalComparisonTests.cs	New test file with comprehensive tests for logical comparison including default value scenarios
src/Microsoft.Performance.SDK.Tests/StringWithCaseInsensitiveComparisonTests.cs	New test file with tests for case-insensitive comparison including default value scenarios
src/Microsoft.Performance.SDK.Tests/AssemblyInfo.cs	New assembly info file with UnitTest attribute marker

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