R1:a9822a37aa5a

Fix a PHP 8.1 unit test failure in Projects

Summary: Ref T13588. This field may be "null" (and is probably never the empty string, but that's a more ambitious fix).

Test Plan: Ran unit tests, got a pass.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21752