PM plugin - fatal error if user is not allowed to add attachment

[Jimmi08]

Bug Description

$this->pmPrefs['attach_class'] has value 254

A normal user doesn't see this field thanks this check:
if (isset($this->pmPrefs['attach_class']) && check_class($this->pmPrefs['attach_class']))

BUT in PHP 8.2. it fails on
foreach (array_keys($_FILES['file_userfile']['size']) as $fid)
because value is NULL and not array

Fatal error: Uncaught TypeError: array_keys(): Argument #1 ($array) must be of type array, null given in ***plugins\pm\pm.php on line 618

Thanks