Fix PHP 8.1 "strlen(null)" exception in DiffusionGitRawDiffQuery

Summary:
strlen() was used in Phabricator to check if a generic value is a non-empty string.
This behavior 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.

Deprecated: strlen(): Passing null to parameter #1 ($string) of type string is deprecated in /home/phd/phabricator/src/applications/diffusion/query/rawdiff/DiffusionGitRawDiffQuery.php on line 38

Closes T15399

Test Plan: Read the code.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15399

Differential Revision: https://we.phorge.it/D25779