Description:
------------
If the array key has null value it isn't extracted to variable. Is it correct behaviour?

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

$arr = [
    'username' => 'mruz',
    'email' => null,
    'age' => 0,
    'website' => false
];

extract($arr);

var_dump(isset($username), isset($email), isset($age), isset($website));


Expected result:
----------------
bool(true) bool(true) bool(true) bool(true)

Actual result:
--------------
bool(true) bool(false) bool(true) bool(true)