See downstream https://phabricator.wikimedia.org/T209449.
The "Bulk Edit" flow works with setContinueOnMissingFields(true), so newRequiredError() errors are ignored. This allows you to apply a transaction which changes the title to "" (the empty string) without actually hitting any errors which the workflow respects.
(Normally, setContinueOnMissingFields(...) workflows only edit properties that can't be missing, like the status of an object, so this is an unusual flow.)
Instead, validate more narrowly:
- Transactions which would remove the title get an "invalid" error, which is respected even under "setContinueOnMissingFields()".
- Then, we try to raise a "missing/required" error if everything otherwise looks okay.