fix: do not write a newline to an empty xmllist (#5514)

prevents issues with processing files when there are empty xmllists

issue only shows when building with very few languages

Co-authored-by: Darragh Elliott <me@delliott.net>
Reviewed-on: #5514
Co-authored-by: delliott <delliott@fsfe.org>
Co-committed-by: delliott <delliott@fsfe.org>

build/fsfe_website_build/phase1/update_xmllists.py

@@ -99,7 +99,7 @@ def _update_for_base(  # noqa: PLR0913
             )
     update_if_changed(
         Path(f"{base.parent}/.{base.name}.xmllist"),
-        ("\n".join(sorted(matching_files)) + "\n"),
+        "\n".join(sorted(matching_files)) + "\n" if matching_files else "",
     )