Easy Digital Downloads - Add download to cart if the cart is empty

gistfile1.php

<?php

/**
 * Add product to cart if cart is empty
 */
function sumobi_edd_add_to_cart_if_empty() {
	
	$download_to_add = 123;

	$cart = function_exists( 'edd_get_cart_contents' ) ? edd_get_cart_contents() : false;
 	
	if ( function_exists( 'edd_is_checkout' ) && edd_is_checkout() && ! $cart ) {
		edd_add_to_cart( $download_to_add );
	}

}
add_action( 'template_redirect', 'sumobi_edd_add_to_cart_if_empty', 9 );