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

Summary:
With PHP 8.1+ it was not possible to view a commit from subversion repositories.
Indeed, if the commit user and/or email 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.

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 T15609

Test Plan:
- Apply D25397 and D25398
- Sign in
- Open a diffusion SVN repository
- Open a commit without user name and or email
- You should not see the same Runtime Exception (unfortunately, there is another one as described in T15610)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15609

Differential Revision: https://we.phorge.it/D25399