Fix PHP 8.1 exception in Conduit: Make "array_fuse(array $list)" accept null as parameter

Summary:
array_fuse in Arcanist is a wrapper for calling array_combine($list, $list).
The latter doesn't accept passing null in PHP 8.2.
Going to /conduit/method/project.create/, entering a name but nothing as members (so we pass null), and calling this method, an exception is thrown.

Thus make array_fuse accept null and return an empty list in such cases.

Closes T15393

Test Plan: Applied this change; afterwards "Method Call Result" page at /api/project.create correctly displayed in the web browser.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15393

Differential Revision: https://we.phorge.it/D25228