Description:
------------
caling empty function with SPL object attribute (arrayaccess) as parameter return always false (is not empty).

Reproduce code:
---------------
class a implements ArrayAccess {

  private $vars;

  public function offsetExists($offset) {
      return (isset($this->vars[$offset]));
  } // offsetExists

  public function offsetGet($offset) {
        return $this->vars[$offset];
    } // offsetGet

  public function offsetSet($offset, $value) {
        $this->vars[$offset] = $value;
    } // offsetSet

  public function offsetUnset($offset) {
        if ($this->offsetExists($offset)) {
            unset($this->vars[$offset]);
        } // if
    } // offsetUnset
}

$x = new a();
$x['key'] = '';
var_dump($x['key']);
var_dump(empty($x['key']));

$y = $x['key'];
var_dump($y);
var_dump(empty($y));


Expected result:
----------------
string(0) "" bool(true) string(0) "" bool(true)

Actual result:
--------------
string(0) "" bool(false) string(0) "" bool(true)