code: func (v *StringIsPresent) IsValid(errors *validate.Errors) {
  	if strings.TrimSpace(v.Field) != "" {
  		return
  	}
  
  	if len(v.Message) > 0 {
  		errors.Add(GenerateKey(v.Name), v.Message)
  		return
  	}
  
  	errors.Add(GenerateKey(v.Name), fmt.Sprintf("%s can not be blank.", v.Name))
  }
docs: // IsValid adds an error if the field is empty.