Description:
------------
ArrayObject itself and child classes work fine until you override offsetExists() method, even with a method returning parent's results. Both isset() and empty() return incorrect results on NULL value set to the pseudo-array.

Reproducible on PHP 5.4.27, Windows 7 x86.

Test script:
---------------
http://pastebin.com/geNadPSA

Expected result:
----------------
array(2) {
  ["ArrayObject"]=>
  array(2) {
    ["empty"]=>
    bool(true)
    ["isset"]=>
    bool(false)
  }
  ["ArrayObjectOverride"]=>
  array(2) {
    ["empty"]=>
    bool(true)
    ["isset"]=>
    bool(false)
  }
}

Actual result:
--------------
array(2) {
  ["ArrayObject"]=>
  array(2) {
    ["empty"]=>
    bool(true)
    ["isset"]=>
    bool(false)
  }
  ["ArrayObjectOverride"]=>
  array(2) {
    ["empty"]=>
    bool(false)
    ["isset"]=>
    bool(true)
  }
}