fix(translation_graph): prevent CI failure when writing translation stats

[dimasd-angga]

Summary

Translated-language Sphinx builds in CI were failing with FileNotFoundError when write_translation_stats tried to open _static/translation_stats.json. The hook assumed the _static directory already existed and that stats should always be written, which isn't true when the build errored before Sphinx materialised the output tree or when no .po files were collected. This PR makes the writer defensive in those two cases and ensures the parent directory is created before opening the file.

Changes

• In _ext/translation_graph.py, early-return from write_translation_stats when Sphinx passes a non-None exception, so a failed build doesn't trigger a second failure inside the build-finished hook.
• Early-return when get_translation_stats() returns no entries, instead of writing an empty stats file for builds that have no translation sources.
• Wrap app.outdir in Path(...) and call out_path.parent.mkdir(parents=True, exist_ok=True) before writing, so the _static directory is guaranteed to exist regardless of build configuration.

Testing

Ran nox -s docs-test locally; the suite completes without the FileNotFoundError previously seen in CI. No new tests were added — the change is a guard inside a Sphinx build-finished callback, and the existing docs build exercises the path.

Notes

The two new early-returns log at info level so it's still visible in CI output when stats are skipped, which should make future debugging easier without changing the build outcome.

Closes #649