PHP 8.5: Fix null array key checking project membership when anonymous
Summary:
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 the key parameter for array_key_exists() is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/infrastructure/storage/lisk/PhabricatorLiskDAO.php:287]
```
Closes T16377
Test Plan:
* PHP 8.5
* Set `/config/edit/policy.allow-public` to `true`
* Create a Maniphest task with View Policy = Public
* Access that task while not logged in
* Get two errors less
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16377
Differential Revision: https://we.phorge.it/D26551