R1:e69b349b1b24

Prevent users from removing task titles with "Bulk Edit"

Summary:
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…