Fix NRE when localpath was null

[adamzip]

#5830

[Copilot]

Pull request overview

Updates DARC’s remote file-fetching logic to avoid a null-reference exception when the optional baseDirectory is not provided, specifically in GetCommonScriptFilesAsync.

Changes:

• Coalesce baseDirectory to an empty path and build a single commonScriptsPath.
• Pass the computed commonScriptsPath into _remoteGitClient.GetFilesAtCommitAsync.
• Update logging to report the computed common scripts path.

[Copilot]

When baseDirectory is null this now becomes UnixPath.Empty, but commonScriptsPath = baseDirectory / ... uses the LocalPath / operator (compile-time type), which will combine empty left + right into a path with a leading / (e.g. /eng/common). Previously the null case used Constants.CommonScriptFilesPath (eng/common) without the leading slash. Consider preserving the old behavior (keep the conditional), or cast/convert to UnixPath before combining so UnixPath's empty-path handling is used.

[adamzip]

    public static UnixPath operator /(UnixPath left, string right)
        => IsEmptyPath(left)
            ? new(right)
            : new(left.Combine(left.Path, left.NormalizePath(right)), false);

UnixPath.Empty combined with another path should not produce any extra slashes according to this implementation

[Copilot]

The stripBaseDirectory branch below uses f.FilePath.TrimStart(baseDirectory).ToString() which does not remove a base-directory prefix; it trims any leading characters contained in baseDirectory (via MemoryExtensions.TrimStart on spans). For base directories like src/arcade, this can corrupt paths (e.g., stripping the leading e from eng/...). Consider replacing that logic with explicit prefix/path-segment removal (e.g., StartsWith baseDirectory + separator and slice/replace only that prefix).