MT | ET | Force users to select at least one ticket or RSVP before submitting the form

tribe_prevent_add_cart_empty.php

<?php

/*
 * Prevents users from adding tickets to cart unless they have selected at least one ticket
 */
function tribe_prevent_add_cart_empty() {
	echo '
	<script type="text/javascript">
		jQuery( document ).ready(function(){
			jQuery( "form.cart" ).each(function(){
				var tribe_empty_form_message = "Please select at least one item before submitting";
				var tribe_cart_submit_button = jQuery( this ).find( "button[type=submit]" )[0];
				var tribe_checkout_fields = jQuery( this ).find( "input[type=number]" );

				tribe_cart_submit_button.setCustomValidity( tribe_empty_form_message );

				tribe_checkout_fields.each(function(){
					jQuery( this ).on( "change", function(){
						for (var i = 0, total_tickets = 0; i < tribe_checkout_fields.length; i++)
							total_tickets = total_tickets + parseInt(tribe_checkout_fields[i].value);

						if( total_tickets > 0 )
							tribe_cart_submit_button.setCustomValidity( "" );
						else
							tribe_cart_submit_button.setCustomValidity( tribe_empty_form_message );
					});
				});
			});
		});
	</script>';

}
add_action( 'tribe_events_single_event_after_the_meta', 'tribe_prevent_add_cart_empty' );