PHP 8.5: Files: Do not check for an empty engine key as array key

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/files/document/render/PhabricatorDocumentRenderingEngine.php:52]
```

Closes T16416

Test Plan: On PHP 8.5, go to a File page in the File application.

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16416

Differential Revision: https://we.phorge.it/D26625

src/applications/files/document/render/PhabricatorDocumentRenderingEngine.php

···
     $engines = PhabricatorDocumentEngine::getEnginesForRef($viewer, $ref);
 
     $engine_key = $this->getSelectedDocumentEngineKey();
-    if (!isset($engines[$engine_key])) {
+    if ($engine_key === null || !isset($engines[$engine_key])) {
       $engine_key = head_key($engines);
     }
     $engine = $engines[$engine_key];