Fix strlen(null) PHP 8.1 error when pushing into a fresh repository

Summary:
Doing a git push into a fresh repository over https fails with a PHP 8.1 strlen(null) error.

This fixes it.

Fixes T15569

Test Plan:

• Create an empty repository on a PHP 8.1 Phorge instance
• Clone it
• Add some data
• git push

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15569

Differential Revision: https://we.phorge.it/D25361