Return empty toctree for missing TOC

[Herrtian]

Closes #253.

Return an empty toctree fragment when a 404 page has no TOC, so templates get a string instead of None.

Checks:

• uv run --with pytest --with defusedxml pytest tests/test_urls.py::test_empty_toctree_returns_empty_string -q
• uv run --with pytest --with defusedxml pytest tests/test_urls.py::test_toctree_urls_notfound_default tests/test_urls.py::test_toctree_links tests/test_urls.py::test_toctree_links_custom_settings -q
• git diff --check