PHP 8.5: Typing a comment: Gracefully handle missing author PHID

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."

Do not add null to the PHID array when there is no author PHID.

```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/infrastructure/query/policy/PhabricatorPolicyAwareQuery.php:525]
```

Closes T16368

Test Plan: On PHP 8.5, go to any existing item (e.g. D1 or T1) and type in the Comment field.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16368

Differential Revision: https://we.phorge.it/D26602

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

···
         break;
     }
 
-    if ($this->getComment()) {
+    if ($this->getComment() && $this->getComment()->getAuthorPHID()) {
       $phids[] = array($this->getComment()->getAuthorPHID());
     }