Returns whether the given element has no content.
An element is considered to have content if it has either child elements or non-whitespace text.
Examples
These <div> elments are all considered empty:
<div></div>
<div> </div>
<div attr="text"></div>
These <div> elements are not considered empty:
<div>
Text
</div>
<div>
<span>Text</span>
</div>
<div>
<span></span>
</div>
