Empty label doesnt work correctly for ObjectSelect

On Form\Element\Proxy.php

```
    if ($this->displayEmptyItem) {
        $options[''] = $this->getEmptyItemLabel();
    }
```

Should be changed to:

$options[] = array('label' => $this->getEmptyItemLabel(), 'value' => '');

Otherwise the emptyLabel wont be displayed properly and rater a NULL is inserted as item...
