[PHP 8.1] Fix Deprecated notice: strtolower(): Passing null to string…

… is deprecated in components/com_ajax/ajax.php on line 39 (#37007)

Fix Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in components/com_ajax/ajax.php on line 39

components/com_ajax/ajax.php

@@ -36,7 +36,7 @@
 $input = $app->input;
 
 // Requested format passed via URL
-$format = strtolower($input->getWord('format'));
+$format = strtolower($input->getWord('format', ''));
 
 // Initialize default response and module name
 $results = null;