Fix doc: include empty values by array_group_by() (#8396)

Author: totoprayogo1916

user_guide_src/source/helpers/array_helper/014.php

@@ -1,6 +1,6 @@
 <?php
 
-$result = array_group_by($employees, ['gender', 'hr.department']);
+$result = array_group_by($employees, ['gender', 'hr.department'], true);
 
 $result = [
     '' => [