code: protected void encodeOptionalAttr(final WidgetBuilder wb, final String attrName, final String value)
                  throws IOException {
          if (value != null) {
              wb.attr(attrName, value);
          }
      }
docs: Encodes an optional attribute to the widget builder specified.
  
  @param wb the WidgetBuilder to append to
  @param attrName the attribute name
  @param value the value
  @throws IOException