preg_match() null exception setting custom user profile image with empty files.viewable-mime-types

Summary:
When `files.viewable-mime-types` is not set, `getViewableMimeType()` passes `null` to `preg_match()` which is deprecated behavior since PHP 8.1.
Only call `preg_match()` when there are some MIME types to compare.

```
ERROR 8192: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/files/storage/PhabricatorFile.php:974]
```

Closes T15710

Test Plan: Go to a user profile and try to upload a custom profile picture in BMP format.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15710

Differential Revision: https://we.phorge.it/D25516

src/applications/files/storage/PhabricatorFile.php

@@ -971,10 +971,13 @@ final class PhabricatorFile extends PhabricatorFileDAO
     // warns you if you don't have complete support.
 
     $matches = null;
+    $ok = false;
+    if ($this->getViewableMimeType() !== null) {
       $ok = preg_match(
         '@^image/(gif|png|jpe?g)@',
         $this->getViewableMimeType(),
         $matches);
+    }
     if (!$ok) {
       return false;
     }