No is_empty filter for nullable relations

class sfWidgetFormFilterDoctrineChoice extends sfWidgetFormDoctrineChoice {

  protected function configure($options = array(), $attributes = array()) {
    $this->addOption('with_empty', true);
    $this->addOption('empty_label', 'is empty');
    $this->addOption('template', '%input%<br />%empty_checkbox% %empty_label%');

    parent::configure($options, $attributes);
  }

  public function render($name, $value = null, $attributes = array(), $errors = array())
  {
    $values = array_merge(array('text' => '', 'is_empty' => false), is_array($value) ? $value : array());

    return strtr($this->getOption('template'), array(
      '%input%'          => parent::render($name, $value, $attributes, $errors),
      '%empty_checkbox%' => $this->getOption('with_empty') ? $this->renderTag('input', array('type' => 'checkbox', 'name' => $name.'[is_empty]', 'checked' => $values['is_empty'] ? 'checked' : '')) : '',
      '%empty_label%'    => $this->getOption('with_empty') ? $this->renderContentTag('label', $this->getOption('empty_label'), array('for' => $this->generateId($name.'[is_empty]'))) : '',
    ));
  }
}

Can you provide your changes as a patch/diff?