Steps to reproduce the issue

I think calling anything with the JEventDispatcher->trigger with an event name and no args OR an empty args array will result in a error.

In my plugin I am calling

$dispatcher = JEventDispatcher::getInstance();
$dispatcher->trigger('onUserAfterLogout'); // trigger('onUserAfterLogout', array()); is also problematic

to get the PlgAuthenticationCookie to do it's job. If the 2nd parameter to trigger is not specified or I pass array(), I will get

Too few arguments to function PlgAuthenticationCookie::onUserAfterLogout(), 0 passed in libraries/joomla/event/event.php on line 70 and exactly 1 expected

if I call $dispatcher->trigger('onUserAfterLogout', array('whatever' => 'whatever')); it will work!

The problem seems to come from the call in JEvent->update(&$args) where the method call_user_func_array is called, PHP seems to NOT assign anything to the arguments of the triggered method if the array is empty.

Expected result

Shouldn't raise an error!

IMO the JEvent should handle this and properly setup the $args array to avoid this situation

Actual result

Too few arguments to function PlgAuthenticationCookie::onUserAfterLogout(), 0 passed in libraries/joomla/event/event.php on line 70 and exactly 1 expected

System information (as much as possible)

Joomla 3.9.3
PHP 7.2 (AFAIK was not problematic under 5.6)
Debian 9 fully updated

Additional comments