Update DiffusionDocumentRenderingEngine to cope with null code coverage in PHP 8.1

Summary:
Update DiffusionDocumentRenderingEngine to cope with null code coverage in PHP 8.1

Fixes T15523

Test Plan: https://my.phorge.site/source/myrepo/browse/master/myfile

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15523

Differential Revision: https://we.phorge.it/D25330