Avoid another preg_replace call with null argument

When a user without rights on a view clicks on a link to a search on that view (eg in Chemistry's Xymon instance all hostnames are links to a database search for that hostname in a view accessible only to IT staff; users have access to Xymon so sometimes click on those links) then PHP 8.1 is throwing an error from a call in dbList.php that has a null parameter. This change checks for null before making the call.