Support make_compound_path concatenating only empty paths.

[anntzer]

Previously codes[i] would fail if codes (i.e. the concatenated path) had length zero.

Split out as preparatory work for #25247 (which introduces empty paths for contour levels that don't actually result in any level being drawn).

PR Summary

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst