`$bot->on($fn)` doesn't execute `$event`

[codeninja-ru]

Bug

The documentation says:

If second closure will return true (or if you are passed null instead of closure), first one will be executed.

Api/src/Client.php

Lines 129 to 143 in a8edf2e

	/**
	* Use this method to add an event.
	* If second closure will return true (or if you are passed null instead of closure), first one will be executed.
	*
	* @param \Closure $event
	* @param \Closure|null $checker
	*
	* @return \TelegramBot\Api\Client
	*/
	public function on(Closure $event, ?Closure $checker = null)
	{
	$this->events->add($event, $checker);
	return $this;
	}

I passed null as the second parameter.

$bot = new \TelegramBot\Api\Client("token");
$bot->on(function($update) {
  // expected to be executed, but it's not
  var_dump(1);
});
$offset = 0;
while (true) {
    $updates = $bot->getUpdates($offset);
    $bot->handle($updates);
    if (count($updates) > 0) {
        $offset = end($updates)->getUpdateId() + 1;
    }
    sleep(5);
}

The $event is never executed.

Expected

The $event expected to be executed when the second parameter is null or absent.