PHP 8.5: Fix null array key in ConpherenceThread::hasAutomaticCapability()

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/conpherence/storage/ConpherenceThread.php:276]
```

Closes T16510

Test Plan: A bit unclear. For me: PHP 8.5, have Conpherence application enabled, go to Feed Transactions at http://phorge.localhost/feed/transactions/

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16510

Differential Revision: https://we.phorge.it/D26775

src/applications/conpherence/storage/ConpherenceThread.php

···
         return false;
     }
 
+    if (!$user->getPHID()) {
+      return false;
+    }
+
     $participants = $this->getParticipants();
     return isset($participants[$user->getPHID()]);
   }