PHP 8.5: Do not use null array keys for comment action dropdown groups

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/transactions/view/PhabricatorApplicationTransactionCommentView.php:597]
```

Closes T16360

Test Plan: Visit a Maniphest task and a Differential revision (the latter has dropdown sub-items!), open the "Add Action..." dropdown, select actions.

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16360

Differential Revision: https://we.phorge.it/D26529

src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php

···
     // Merge options into groups.
     $groups = array();
     foreach ($action_map as $key => $item) {
-      $group_key = $item['groupKey'];
+      $group_key = $item['groupKey'] ?? '';
       if (!isset($groups[$group_key])) {
         $groups[$group_key] = array();
       }