Model\Annotations fix nullable annotation

- nullable allows column to a null value

Because it is passed in https://github.com/ovr/framework/blob/b9354b2efe16dcfe24850eae4a391e8b5a92f14e/src/Db/Model/Annotations/Metadata.php#L139

```php
/**
* @Column(name="lastname", type="string", length=45, nullable=true)
* @var string
*/
public $lastname;
```

Without it, $lastname cant be null