code: @Override
    public final org.jdom2.Element addContent(Content content) {
      // ignore empty elements
      if (content == null) {
        return null;
      }
      return super.addContent(content);
    }
docs: Appends the child to the end of the element's content list
  @param content Child to append to end of content list. Null values are ignored.
  @return The element on which the method was called.