Description:
------------
Behavior \JsonSerializable different from json_encode if contains json_decode(null, true) in PHP 7.

Test script:
---------------
https://3v4l.org/UraSC

<?php

$result = json_encode(['end' => json_decode(null, true)]);
var_dump($result);

class A implements \JsonSerializable
{
    function jsonSerialize()
    {
        return ['end' => json_decode(null, true)];
    }
}
$a = new A();
$toJsonData = $a->jsonSerialize();
$result = json_encode($a);
var_dump($result);

$result = json_encode($toJsonData);
var_dump($result);