Added continueIfEmpty annotation to Zend Form

```php
$this->continueIfEmpty = (isset($data['value'])) ? $data['value'] : false;

if (!is_bool($continueIfEmpty)) {
    $filter   = new BooleanFilter();
    $this->continueIfEmpty = $filter->filter($this->continueIfEmpty);
}

```

Small improvment, it's much easier to read IMHO.