[NFR]Add param allow_empty to \Phalcon\Mvc\Model::validate

Before
```php
if (!empty($this->idcard)) {
	$this->validate(new Uniqueness(array(
		'field' => 'idcard',
		'message' => 'exists'
	)));
}
```

Now
```php
$this->validate(new Uniqueness(array(
	'field' => 'idcard',
	'message' => 'exists'
)), TRUE);
```