PHP8.1 fix for DiffusionServeController serveRequest()

Summary:
When a 'git pull' is done to an https git URL, the $_SERVER variables PHP_AUTH_USER and PHP_AUTH_PW will be unset, causing PHP 8.1 to throw strlen(null) errors.

This update fixes the issue by defaulting the values to '', which results in $have_user and $have_pass having false values as desired.

Fixes T15520

Test Plan: arc unit

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15520

Differential Revision: https://we.phorge.it/D25327