[ticket/13134] Add core event to the root of the function display_forums()

<a href="https://tracker.phpbb.com/browse/PHPBB3-13134">PHPBB3-13134</a>.
Yes, but here https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/includes/functions_display.php#L43 it's being defined as array if empty, so it should be an array anywhere below. So I'm not sure.