Fix a PHP 8.1 deprecated use of strlen with a NULL argument on commit page

Summary:
With PHP 8.1+ it is not possible to view a commit if the author field is not properly defined
Indeed, if the commit author is not properly defined, strlen(null) is called, causing a deprecation warning, elevated to exception.
Using strlen() to check string validity is deprecated since PHP 8.1. Phorge adopts phutil_nonempty_string() as a replacement.

Fix T15628

Test Plan:

• Push a new commit on a subversion repository (since T15629 is not yet addressed)
• Visualize the commit
• You should not get a RuntimeException

Reviewers: O1 Blessed Committers, Sten, valerio.bozzolan

Reviewed By: O1 Blessed Committers, Sten, valerio.bozzolan

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

Maniphest Tasks: T15628

Differential Revision: https://we.phorge.it/D25422