[9.x] Fix missing return

[wfeller]

No description provided.

[GrahamCampbell]

Thanks for the PR. What exactly does this fix, please?

[wfeller]

use \Illuminate\Mail\Attachment;
use \Illuminate\Notifications\Messages\MailMessage;

$message = new MailMessage;

// this should return the message but returns null, which prevents from chaining other message methods
$message->attach(
    Attachment::fromStorageDisk('local', 'path.pdf')
);

[timacdonald]

I've verified this is indeed a fix and added a test for good measure that fails without this change.

Thanks for the PR @wfeller.