Add polyfill for string.IsNullOrWhiteSpace(...)

[Tyrrrz]

No description provided.

[Copilot]

Pull Request Overview

This PR adds a polyfill for string.IsNullOrWhiteSpace(...) to support .NET Framework versions prior to 4.0 (such as .NET Framework 3.5), where this method was not yet available. The implementation follows the existing polyfill pattern used throughout the codebase.

Key Changes:

• Added String.cs polyfill in the NetCore10 directory with conditional compilation for pre-.NET 4.0 frameworks
• Implemented IsNullOrWhiteSpace as a static extension method that checks for null or whitespace-only strings

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (34464d9) to head (6eefbb2).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@     Coverage Diff      @@
##   master   #28   +/-   ##
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