Fix Diviner strlen(null) error when clicking on a link

Summary:
Fix issue whereby clicking on a link in Diviner (eg https://my.phorge.site/diviner/find/?name=Differential_User_Guide&type=article&jump=1) results in a strlen(null) error under PHP 8.1

Fixes T15635

Test Plan:

• Go into the Diviner differential page /book/phorge/article/differential/
• Click on the link to Differential User Guide: Inline Comments

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15635

Differential Revision: https://we.phorge.it/D25433