R1:7702772b2b86

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]…