Replace text with Category (if else)

I have a custom field that the client can or cannot fill in, if they don’t fill it in i want to replace it with the post’s category.

I have this code below to insert the custom field, now i just need an else statement after it to insert category if the $painter field IS blank.

<?php
// Show artist if we have one
if ($painter != "") {
echo '<h3>' . $painter . '</h3>';
}
?>

Any ideas?
Thanks!