EM - Make Spaces Dropdown Required

<?php
function stonehenge_em_add_empty_to_spaces() {
    // Only add the script if this is a single event and has bookings enabled.
    global $EM_Event;
    if( is_singular('event') && '1' === $EM_Event->rsvp ) {
        ?><script>
            jQuery.noConflict();
            jQuery(".em-ticket-select").prepend("<option value='0' selected='selected' disabled='disabled'>--</option>");
            (jQuery);
        </script><?php
    }
}
add_action('em_booking_form_ticket_footer', 'stonehenge_em_add_empty_to_spaces');