MediaWiki sometimes creates dummy "null" revisions in the page history to mark events that affect the page's state, but not the pages content. Examples include move/rename and page protection changes.
So far, application logic had to "manually" insert a revision and update the page table to achieve this. However, this is seems rather brittle, and leads to inconsistencies wrt what updates are performed when a dummy revision is created.
For this reason, dummy revisions should be created using the new PageUpdater interface, simply by not supplying any new content before saving the update. However, this would usually trigger a "null edit", which re-applies secondary data updates but does not create a new revision. So PageUpdater will need a mode that forces the creation of empty dummy revisions.