Fix PHP 8.5 null array key creating an Almanac Service

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/almanac/controller/AlmanacServiceEditController.php:19]
```

Closes T16433

Test Plan: On PHP 8.5, go to http://phorge.localhost/almanac/service/edit/form/default/

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16433

Differential Revision: https://we.phorge.it/D26653

src/applications/almanac/controller/AlmanacServiceEditController.php

···
 
       $service_type = $request->getStr('serviceType');
       $service_types = AlmanacServiceType::getAllServiceTypes();
-      if (empty($service_types[$service_type])) {
+      if ($service_type === null || empty($service_types[$service_type])) {
         return $this->buildServiceTypeResponse($list_uri);
       }