empty values New in Symfony version 2.3 .
empty values are also supported if the expanded option is set to true like Symfony 2.3 Documentation
public function buildForm(FormBuilderInterface $builder, array $options) {
$builder
->add('manifestations', 'entity', array(
'class' => 'PrifProtocoleBundle:Manifestation',
'multiple' => true,
'expanded' => true,
'property' => 'name',
'empty_value' => 'All',
'required' => false,));
}