See #7225 (comment)

See also docs for header(): https://www.php.net/manual/en/function.header.php

The $response_code parameter is define as follows:

Forces the HTTP response code to the specified value. Note that this parameter only has an effect if the header is not empty.

Note empty here. So 0 and null should work the same since they are both empty.

Note also the function signature:

header(string $header, bool $replace = true, int $response_code = 0): void

The default value of $response_code is 0.