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 pages (Differential, Maniphest, Files, Paste, and Ponder) finally rendered in web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15359

Differential Revision: https://we.phorge.it/D25198