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

Summary:
This call is preventing users to browse Subversion repositories.
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 T15608

Test Plan:

• Sign in
• Open a Diffusion SVN repository
• You should see the repository instead of getting a Runtime Exception

Reviewers: O1 Blessed Committers, Sten, avivey

Reviewed By: O1 Blessed Committers, Sten, avivey

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

Maniphest Tasks: T15608

Differential Revision: https://we.phorge.it/D25398