J!4, PHP8.1. Deprecated: htmlspecialchars(): Passing null to parameter #1 ($string)...

Fixes PHP8.1 message in Joomla 4 frontend. OSModule v2.21.

Deprecated: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in /modules/mod_osmod/mod_osmod.php on line 22.