code: public static <I> InputValidator<I> checkNotNull(@Nullable I input, ValidationMessage message)
  			throws ValidationException {
  		if (input == null) {
  			throw new ValidationException(message.format(input));
  		}
  		return new InputValidator<>(input);
  	}
docs: Checks that an input value is not {@code null}.
  
  @param <I> the actual input type.
  @param input the input value to check.
  @param message the validation message to create in case the validation fails.
  @return a new {@linkplain InputValidator} instance for further validation steps.
  @throws ValidationException if the validation fails.