Description:
------------
Seems logical that an error should be thrown since that index doesnt exist in the array. instead, the index is silently added with a value of null. All versions of php >5.4 and all versions of HHVM are affected.

Repro: https://3v4l.org/UNIe4

Test script:
---------------
<?php

$data = [
    'existant_key'=>[]
];

foreach($data['non_existant_key'] as &$row)
{
}

print_r($data);

?>
https://3v4l.org/UNIe4

Expected result:
----------------
undefined index

Actual result:
--------------
array is silently modified to include the non existant key