Add method chaining support to NullDriver

[imacrayon]

I'm tagging subscribers in a few places with code like this:

$hash = Newsletter::getApi()->subscriberHash($user->email);
Newsletter::getApi()->post("lists/123/members/{$hash}/tags", [
    'tags' => [['name' => 'My Tag', 'status' => 'active']],
]);

In a local or testing environment I have NEWSLETTER_DRIVER=null set in my .env. However this causes getApi()->subscriberHash() to throw an exception since getApi() returns null when a Newsletter driver is not set.

This PR ensures NullDriver return itself when a method is called so that method calls can be chained. getApi()->subscriberHash() will now return an instance of NullDriver instead of throwing.