rARCe4fd31ec024e

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…