Fix a PHP 8.1 deprecated use of strlen with a NULL argument

Summary:
This call prevents users to view a commit in subversion repositories
Indeed, if commiter and/or author field is not properly defined strlen is call with a NULL argument.
Using strlen to check string validity is deprecated since PHP 8.1, phorge adopts phutil_nonempty_string() as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Fix T15610

Test Plan:

• Sign in (if needed)
• Open a diffusion SVN repository
• Open a commit without user name and or email
• You should be able to view the commit

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15610

Differential Revision: https://we.phorge.it/D25400