code: @Override
    public final org.jdom2.Element addContent(Collection collection) {
      // ignore empty elements
      if (collection == null) {
        return null;
      }
      return super.addContent(collection);
    }
docs: Appends all children in the given collection to the end of the content list. In event of an exception
  during add the original content will be unchanged and the objects in the supplied collection will be unaltered.
  @param collection Collection to append. Null values are ignored.
  @return the element on which the method was called