Handle empty argv in RootCommand.ExecutablePath

[marko1olo]

Fixes #2812.

RootCommand.ExecutablePath currently indexes Environment.GetCommandLineArgs()[0] directly. Some NativeAOT hosts can report an empty argv array, which makes constructing or otherwise touching RootCommand throw IndexOutOfRangeException before user code can run.

This change routes executable path extraction through a small helper and falls back to string.Empty when the runtime exposes no argv entries. The normal non-empty argv behavior is unchanged.

Validation:

• git diff --check
• dotnet test src/System.CommandLine.Tests/System.CommandLine.Tests.csproj --filter RootCommandTests --no-restore was attempted locally but blocked because global.json requires SDK 11.0.100-preview.4.26210.111 and this machine has 10.0.202 installed.

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