html,javascript:向当前空元素追加一个新元素

document.querySelectorAll('div').forEach(div => {
  if (div.innerHTML.trim() === '') div.textContent = 'Empty';
});

<div></div>
<div>


</div>

<div>I am not empty</div>

var checkDivs = () => {
  document.querySelectorAll('#container').forEach((el)=>{
    el.innerText = (el.innerText) ? el.innerText:'EMPTY';
  })
}
checkDivs();

<div id="container">


</div>
if the container has only whitespaces then:
<div id="container">EMPTY</div>

VS

<div id="container">I am not empty</div>
if the container has content (no need to append EMPTY):
<div id="container">I am not empty</div>