Currently bulk publishing raises an error if you include pages that
exist in the database without any revisions. This commit ensures
that a revision exists before the page is published.
Fixes issue 8187.
As per discussion on https://github.com/wagtail/wagtail/discussions/7732, the changes in the upcoming release (sidebar redesign, removal of hallo.js, module reorganisation) warrant a version bump to 3.0 if we're following semantic versioning.
