PHP 8.5: Maniphest: Do not pass a null array entry to loadHandles()
Summary:
`PhabricatorHandlePool::newHandleList(array $phids)` does `$this->handles[$phid]`. The passed array sometimes has two items, the first being a null element. `PhabricatorHandlePool::newHandleList` is called by `PhabricatorUser::loadHandles()` is called by `ManiphestTaskDetailController::buildCurtain()` which can pass null as the first element in `$handles = $viewer->loadHandles(array($owner_phid, $author_phid));`.
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."
```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/phid/handle/pool/PhabricatorHandlePool.php:29]
```
Closes T16359
Test Plan: Visit a Maniphest task without an assignee set in PHP 8.5. Get one error less.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16359
Differential Revision: https://we.phorge.it/D26528