Description:
------------
I'm had this script in older version of php and there was not any error. When I upgraded to PHP 7.0.6 this problem occurred. 

Test script:
---------------
class Json implements JsonSerializable
{
    public function jsonSerialize()
    {
        json_decode("");

        return ["test" => "test"];
    }
}

$test = new Json;

header('Content-Type: application/json');
echo json_encode($test);


Expected result:
----------------
{"test":"test"}

Actual result:
--------------
""