Fix an instance of strlen(null)

Summary: Ref Q112, ref T15064.

Test Plan:
Enter the text [[hello]] in a remarkup field - without this change, it causes a deprecation warning in the log.

This doesn't happen if the content looks like a valid URL - this is the "Wiki Link" syntax.

Reviewers: bekay, O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15064

Differential Revision: https://we.phorge.it/D25539