code: public static StringValidator checkNotEmpty(@Nullable String input, ValidationMessage message)
  			throws ValidationException {
  		if (input == null || input.length() == 0) {
  			throw new ValidationException(message.format(input));
  		}
  		return new StringValidator(input);
  	}
docs: Checks that an input value is not empty.
  
  @param input the input value to check.
  @param message the validation message to create in case the validation fails.
  @return a new {@linkplain StringValidator} instance for further validation steps.
  @throws ValidationException if the validation fails.