Updated:
You can customize a bit here, below is my tweak:
1 - Open file: /components/com_dtregister/templates/default/views/event/list_row.php
2 - Go to approx line 384:
<?php if ($spots_column) { ?>
change it to:
if ($spots_column && (isset($eventTable->_eventdetail->spots) && $eventTable->_eventdetail->spots)) { ?>
I attach this file here so you can see the difference.