Fix deprecation warning (PHP 8.1+) (joomla#39881)

Fix deprecation warning with PHP 8.1+

If a replacement string is used, but no 'quicktask' is defined `$params->get('menu-quicktask')` will return `null` which is deprecated in PHP 8.1+.

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in administrator/components/com_menus/src/Helper/MenusHelper.php on line 894

administrator/components/com_menus/src/Helper/MenusHelper.php

@@ -891,7 +891,7 @@ protected static function parseXmlNode($node, $replace = [])
             $item->link    = str_replace("{sql:$var}", $val, $item->link);
             $item->class   = str_replace("{sql:$var}", $val, $item->class);
             $item->icon    = str_replace("{sql:$var}", $val, $item->icon);
-            $params->set('menu-quicktask', str_replace("{sql:$var}", $val, $params->get('menu-quicktask')));
+            $params->set('menu-quicktask', str_replace("{sql:$var}", $val, $params->get('menu-quicktask', '')));
         }
 
         $item->setParams($params);