[3.13] gh-122044: Don't error during gitignore filtering with no files (GH-122045) (#122355)

Co-authored-by: Seth Michael Larson <seth@python.org>

Tools/build/generate_sbom.py

@@ -108,6 +108,10 @@ def filter_gitignored_paths(paths: list[str]) -> list[str]:
 
         '.gitignore:9:*.a    Tools/lib.a'
     """
+    # No paths means no filtering to be done.
+    if not paths:
+        return []
+
     # Filter out files in gitignore.
     # Non-matching files show up as '::<whitespace><path>'
     git_check_ignore_proc = subprocess.run(