Database\ActiveRow::offsetExists() a NULL

````php
$array = array('foo' => NULL);
$obj = new \ArrayObject($array);
dump($obj->offsetExists('foo')); //TRUE
dump(array_key_exists('foo', $array)); //TRUE
dump(isset($array['foo'])); //FALSE

$obj = new \ActiveRow($array, $selection);
dump($obj->offsetExists('foo')); //FALSE
````