Fix PHP8.1 strlen(null) error in DivinerArticleAtomizer.php

Summary:
Fix the strlen(null) error in DivinerArticleAtomizer.php

Fixes T16012

Test Plan:
Create Test.book and test.md as per T16012, then run:

/phorge/bin/diviner generate --book Test.book

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16012

Differential Revision: https://we.phorge.it/D25907