handle case where options.marked is empty

little fix for when no marked options are set.  see https://github.com/helpers/handlebars-helper-compose/commit/c1a3187418963ec2d2d45239687e2b9d47f44157#commitcomment-5137128 and https://github.com/helpers/handlebars-helper-compose/issues/12#issuecomment-33143179