PHP 8.5: Tests: Fix tests in PhabricatorEnvTestCase
Summary:
Setting null as an array key is deprecated since PHP 8.5 per https://www.php.net/releases/8.5/en.php: "Using null as an array offset or when calling array_key_exists() is now deprecated. Use an empty string instead."
According to https://web.archive.org/web/20171001040455/https://www.php.net/manual/en/language.types.array.php, `Null will be cast to the empty string, i.e. the key null will actually be stored under ""` at least since PHP 7.2, which we require.
Closes T16495
Test Plan: Run `../arcanist/bin/arc unit ./src/infrastructure/env/__tests__/PhabricatorEnvTestCase.php` in PHP 8.5
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16495
Differential Revision: https://we.phorge.it/D26748