Phalcon PHP validation Uniqueness but allow empty values

I have the following validation:

use Phalcon\Validation;
use Phalcon\Validation\Validator\Uniqueness;

class Users extends BaseModel {
    public function validation() {
        $validator = new Validation();
        $validator->add('some_data', new Uniqueness([
                    'message' => 'this field must be unique or epmty'
        ]));
        // some other rules (...)
        return $this->validate($validator);
    }
}

The question is how to allow pass empty data. I'd like to save in database NULL if data is empty or unique value if it's passed.

You should just be able to do the following in your model class:

public function validation()
    {
        $validator = new Validation();
        $validator->add(
            'example',
            new Uniqueness(
                [
                    'message' => 'Example must be unique',
                    'allowEmpty' => true,
                ]
            )
        );
        return $this->validate($validator);
    }