R1:3819f7ff196e

Fix PHP 8.1 "ltrim(null)" exception which blocks rendering five applications' Configure pages

Summary:
Since PHP 8.1, passing a null string to `ltrim(string $string, string $characters)` is deprecated.

Thus do not check for `$path = null` but check for `$path = ''` before passing `$path` as the `$string` parameter to `ltrim()`, like src/applications/settings/panel/PhabricatorSettingsPanel.php already does.

Closes T15359

Test Plan: Applied this change (on top of D25197) and five applications' Configure…