d5c2139ce4
When running with the linkcheck builder,
app.builder.default_translator_class is None, making us throw an
exception and give up.
We don't need the bootstrap extension in this case, so just do nothing
instead.
Fixes: f72033bb70 ("docs: add bootstrap extension")
Reviewed-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25585>