Fix a PHP 8.1 deprecated use of strlen with a NULL argument in commit parser

Summary:
With PHP 8.1+ it is not possible to import a commit if the commiter field is not properly defined
Indeed, if the committer 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 T15629

Test Plan:

• Push a commit to an observed subversion repository
• Import it via the phorge/bin/repository reparse COMMIT_ID --importing
• The commit should be properly imported and available in Diffusion

Reviewers: O1 Blessed Committers, Sten, valerio.bozzolan

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

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

Maniphest Tasks: T15629

Differential Revision: https://we.phorge.it/D25423