转换Symfony形式的空值

我有一个带有选择框的表单，该字段有一个空值属性。我想翻译它，但添加translation_domain不会改变任何事情。

<?php
namespace Devell'HowFolderBundle'Form;
use Symfony'Component'Form'AbstractType;
use Symfony'Component'Form'FormBuilderInterface;
class NoteType extends AbstractType
{
public function buildForm(FormBuilderInterface $builder, array $options)
    {
    $builder
        ->add('name', 'text')
        ->add('body', 'textarea');

    $categoryChoices = array();
    $builder->add('category', 'entity', array(
        'class'         => 'HowFolderBundle:Category',
        'empty_value'   => 'note.form.category.choose',
        'translation_domain' => 'HowFolderBundle'
    ));
    }
public function getName()
{
    return "note";
}
}