[2.2] [Form] Empty value missing for expanded single-choice field with required set to false

When creating an expanded, single-choice field that is not required

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

an entry for the empty value should be added, but is not.
