Is it possible to have a HTML SELECT/OPTION value as NULL using PHP?

All you need is a check on the post side of things.

if(empty($_REQUEST['type_id']) && $_REQUEST['type_id'] != 0)
    $_REQUEST['type_id'] = null;

No, POST/GET values are never null. The best they can be is an empty string, which you can convert to null/'NULL'.

if ($_POST['value'] === '') {
    $_POST['value'] = null; // or 'NULL' for SQL
}