code: function filterEmptyAtRules(css) {
      /* Filter media queries with no remaining rules */
      css.walkAtRules((atRule) => {
          if (atRule.name === 'media' && atRule.nodes.length === 0) {
              atRule.remove();
          }
      });
  }
docs: Filter rules with no selectors remaining
  @param  {Object} css             The postcss.Root node
  @return {Array}