[Form] Choice field with radio buttons can't have an empty value

When creating an expanded, single choice field with an empty choice

```
$form = $this->createFormBuilder()
    ->add('color', 'choice', array(
        'choices' => array(
            '' => 'Empty',
            '990000' => 'Red',
            '009900' => 'Green',
        ),
        'expanded' => true,
    ))
    ->getForm()
;
```

I get the error

> Form with empty name can not have parent form.
