Description:
------------
If $o = new ArrayObject(); $o['x'] = null;
the result for isset($o['x']) it should be false, instead true is given

Reproduce code:
---------------
<?php
$a = array();
$a['x'] = null;
var_dump(isset($a['x']));
var_dump(empty($a['x']));
var_dump(array_key_exists('x', $a));

$o = new ArrayObject();
$o['x'] = null;
var_dump($o->offsetExists('x'));
var_dump(isset($o['x']));
var_dump(empty($o['x']));
var_dump(array_key_exists('x', $o));
?>

Expected result:
----------------
bool(false)
bool(true)
bool(true)
bool(true)
bool(false)
bool(true)
bool(true)

Actual result:
--------------
bool(false)
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)