This is mostly about adding return types to methods that implement PHP
interfaces, and not passing null to core functions that want a string.
After this patch, and an update to return types in RemexHtml,
tests/phpunit/integration/ has no more errors than in PHP 8.0.
Bug: T289879
Bug: T289926
Change-Id: Ia424f5cc897070f4188ae126b5bf6a1f552db0e1