code: func fieldIsNil(f reflect.Value) bool {
  	if n := nulls.New(f.Interface()); n != nil {
  		return n.Interface() == nil
  	}
  	if f.Kind() == reflect.Interface || f.Kind() == reflect.Ptr {
  		return f.IsNil()
  	}
  	return f.Interface() == nil
  }
docs: // fieldIsNil validates if a field has a nil reference. Also
  // it validates if a field implements nullable interface and
  // it has a nil value.