[msbuild/tools] Add netstandard2.0 helper string functions.

[rolfbjarne]

• Add a 'StringUtils.IsNullOrEmpty' with proper nullability attributes, because the netstandard2.0 version doesn't have it.
• Add a 'string.EndsWith(char)' overload.

[Copilot]

Pull request overview

This PR improves cross-target compatibility for the tooling/MSBuild code by adding small netstandard2.0-friendly string helpers in tools/common, and updating call sites to use them so nullable flow analysis works consistently across target frameworks.

Changes:

• Added StringUtils.IsNullOrEmpty annotated with [NotNullWhen(false)] to improve nullable flow analysis in netstandard2.0 builds.
• Added a string.EndsWith(char) extension method for non-NET builds, and simplified a call site to use the char overload.
• Updated multiple call sites to use StringUtils.IsNullOrEmpty instead of string.IsNullOrEmpty.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tools/dotnet-linker/LinkerConfiguration.cs	Uses StringUtils.IsNullOrEmpty for nullable-friendly optimize-flags handling.
tools/common/StringUtils.cs	Introduces IsNullOrEmpty helper and a non-NET EndsWith(char) extension; refactors parsing to use the helper.
tools/common/StaticRegistrar.cs	Switches nullable checks on single_assembly to StringUtils.IsNullOrEmpty.
tools/common/Assembly.cs	Switches nullable/optional metadata checks to StringUtils.IsNullOrEmpty.
tools/common/Application.cs	Switches RuntimeIdentifier check to StringUtils.IsNullOrEmpty; adds an early-return guard in ParseRegistrar.
msbuild/Xamarin.MacDev.Tasks/Tasks/CompileAppManifest.cs	Uses StringUtils.IsNullOrEmpty to make minimumOSVersionInManifest nullable flow consistent across TFMs.

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✅ API diff for current PR / commit

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

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Hash: 4e9bfbefa07e40212875e895144ba82f330ccca5 [PR build]

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✅ [PR Build #4e9bfbe] Build passed (Build macOS tests) ✅

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[vs-mobiletools-engineering-service2]

🚀 [CI Build #4e9bfbe] Test results 🚀

Test results

✅ All tests passed on VSTS: test results.

🎉 All 175 tests passed 🎉

Tests counts

✅ cecil: All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (macOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (Multiple platforms): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (tvOS): All 1 tests passed. Html Report (VSDrops) Download
✅ framework: All 2 tests passed. Html Report (VSDrops) Download
✅ fsharp: All 4 tests passed. Html Report (VSDrops) Download
✅ generator: All 5 tests passed. Html Report (VSDrops) Download
✅ interdependent-binding-projects: All 4 tests passed. Html Report (VSDrops) Download
✅ introspection: All 6 tests passed. Html Report (VSDrops) Download
✅ linker: All 44 tests passed. Html Report (VSDrops) Download
✅ monotouch (iOS): All 16 tests passed. [attempt 2] Html Report (VSDrops) Download
✅ monotouch (MacCatalyst): All 18 tests passed. Html Report (VSDrops) Download
✅ monotouch (macOS): All 18 tests passed. Html Report (VSDrops) Download
✅ monotouch (tvOS): All 16 tests passed. Html Report (VSDrops) Download
✅ msbuild: All 2 tests passed. Html Report (VSDrops) Download
✅ sharpie: All 1 tests passed. Html Report (VSDrops) Download
✅ windows: All 3 tests passed. Html Report (VSDrops) Download
✅ xcframework: All 4 tests passed. Html Report (VSDrops) Download
✅ xtro: All 1 tests passed. Html Report (VSDrops) Download

macOS tests

✅ Tests on macOS Monterey (12): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Ventura (13): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sonoma (14): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Sequoia (15): All 5 tests passed. Html Report (VSDrops) Download
✅ Tests on macOS Tahoe (26): All 5 tests passed. [attempt 2] Html Report (VSDrops) Download

Linux Build Verification

✅ Linux build succeeded

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Hash: 4e9bfbefa07e40212875e895144ba82f330ccca5 [PR build]