if else for custom $curauth-> field

A combination of get_queried_object_id to fetch the current user ID and get_user_meta to retrieve the banner should do the trick.

<?php
// somewhere in your theme's author.php

if ($img = get_user_meta(get_queried_object_id(), 'user_banner', true)) {
   // they uploaded an image, use it
} else {
   // they did not upload the image, show the default
}