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