Preserve whitespace paths in static web asset tasks

[OvesN]

Context

Whitespace-only paths are valid on Unix, but only when the downstream file operations accepted them before task migration. In previous task migration the chose of using "isNullOrWhitespace" over "isNullOrEmpty" guard was done incorrectly in some cases.

Changes Made

Use IsNullOrEmpty for four direct-write tasks.
Keep IsNullOrWhiteSpace in ConcatenateCssFiles because its directory creation previously rejected whitespace, so absolutizing it would change behavior.

Testing

• Separate Windows failure and Unix success tests

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[Copilot]

Pull request overview

Updates Static Web Assets MSBuild tasks to preserve whitespace-only relative paths (valid on Unix) by switching path checks from IsNullOrWhiteSpace to IsNullOrEmpty for direct-write tasks, while keeping ConcatenateCssFiles’s stricter behavior to avoid changing its historical failure mode.

Changes:

• Switch several tasks’ path absolutization guards to string.IsNullOrEmpty(...) so whitespace-only paths are still resolved via TaskEnvironment.GetAbsolutePath(...).
• Add OS-specific tests that validate whitespace-only path behavior: fail on Windows, succeed (and resolve relative to TaskEnvironment.ProjectDirectory) on Unix.
• Refactor existing multi-threading tests to share assertions/helpers and reduce duplication.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/StaticWebAssetsGeneratePackagePropsFileMultiThreadingTest.cs	Adds Windows-failure vs Unix-success coverage for whitespace BuildTargetPath and refactors shared assertions.
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsPropsFileMultiThreadingTest.cs	Adds OS-specific whitespace TargetPropsFilePath tests and refactors assertion logic.
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetsManifestMultiThreadingTest.cs	Adds OS-specific whitespace Manifest/Cache path tests ensuring resolution against TaskEnvironment.ProjectDirectory.
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/GenerateStaticWebAssetEndpointsPropsFileTest.cs	Adds OS-specific whitespace TargetPropsFilePath tests and factors task creation into a helper.
test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/ConcatenateCssFilesMultiThreadingTest.cs	Adds regression test ensuring whitespace OutputFile still fails (preserving pre-migration behavior) and refactors assertions.
src/StaticWebAssetsSdk/Tasks/StaticWebAssetsGeneratePackagePropsFile.cs	Uses IsNullOrEmpty to allow whitespace-only BuildTargetPath to be absolutized.
src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsPropsFile.cs	Uses IsNullOrEmpty to allow whitespace-only TargetPropsFilePath to be absolutized.
src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetsManifest.cs	Uses IsNullOrEmpty so whitespace-only Manifest/Cache paths are treated as configured and absolutized.
src/StaticWebAssetsSdk/Tasks/GenerateStaticWebAssetEndpointsPropsFile.cs	Uses IsNullOrEmpty to allow whitespace-only TargetPropsFilePath to be absolutized.