PHP 8.5: PhabricatorUserLogView: Do not add missing actor PHID to array
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 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 T16403
Test Plan: Go to http://phorge.localhost/dashboard/panel/edit/?panelType=query and create a query panel with Search For = Account Activity; open the resulting panel
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16403
Differential Revision: https://we.phorge.it/D26599