code: function isEmpty(node) {
    const len = nodeLength(node);
  
    if (len === 0) {
      return true;
    } else if (!isText(node) && len === 1 && node.innerHTML === blankHTML) {
      // ex) <p><br></p>, <span><br></span>
      return true;
    } else if (lists.all(node.childNodes, isText) && node.innerHTML === '') {
      // ex) <p></p>, <span></span>
      return true;
    }
  
    return false;
  }
docs: returns whether node is empty or not.
  
  @param {Node} node
  @return {Boolean}