[3.10] [PHP 8.1] Fixes mod_menu/menu.php type null instead of string

Fixes Deprecated: trim(): `Passing null to parameter #1 ($string) of type string is deprecated in administrator/modules/mod_menu/menu.php on line 383`

administrator/modules/mod_menu/menu.php

@@ -380,7 +380,7 @@ protected function preprocess($items)
 			}
 
 			// Exclude if link is invalid
-			if (!in_array($item->type, array('separator', 'heading', 'container')) && trim($item->link) === '')
+			if (!in_array($item->type, array('separator', 'heading', 'container')) && trim((string) $item->link) === '')
 			{
 				continue;
 			}