docs: cope with removal of language=None in Sphinx 5.0.0

docs: cope with removal of language=None in Sphinx 5.0.0

avoid a few ugly errors that we get here since basing of the Debian
Bookworm release, which is the first to ship a sphinx version newer
than 5.0, which removed support for allowing None as language [0]

[0]: https://www.sphinx-doc.org/en/master/changes.html#release-5-0-0-released-may-30-2022

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>