Description:
------------
Whenever a function (global or class method) returns a reference to an array element which is not set, the value of that element is set to null.

Reproduce code:
---------------
 $data = array();


 function &get_data($key) {
     global $data;
     return $data[$key];
 }

 print_r($data);
 $result = &get_data('TEST');
 echo $result;
 print_r($data);


Expected result:
----------------
The expected result would be that the value of the unset element would not be set and the following warning would be produced:
Undefined index:  TEST in ...

Actual result:
--------------
Array
(
)
Array
(
    [TEST] =>
)