code: public function validateAttribute($model, $attribute)
      {
          if ($this->isEmpty($model->$attribute)) {
              if ($this->value instanceof \Closure) {
                  $model->$attribute = call_user_func($this->value, $model, $attribute);
              } else {
                  $model->$attribute = $this->value;
              }
          }
      }
docs: {@inheritdoc}