Avoid calling preg_match with a null argument in htmlBase

When going straight to a search on a list view (eg from a link in Xymon) an error is logged on PHP 8.1:

Deprecated:  preg_match(): Passing null to parameter #2 ($subject) of
type string is deprecated in
/var/www/hotwire3/hotwire3/chemistry/classes/htmlBase.php on line 417

This updates the code to check if the argument is null before calling the preg_match function.