Populate Website URL at registration time

A few things come to mind…

1) Do users have to activate their accounts? If they do, it’s quite possible that users who haven’t activated yet are the users who don’t return an author URL

2) If this is occurring for all users, that shouldn’t matter since you already know what the expected URL is going to be.

<?php $author_url = bloginfo('url') . '/author/' . $display_name; ?>

Hope this helps. Good luck!