I know it has been more than a year, but just in case someone else has the same question. It depends on the kata creator, and most of the time, if validation is needed, it is specified in the description. However, in this specific case, it is not, and there are cases with negative numbers that need to be considered as 0, and values over 255 that need to be considered as 255. One thing that is specified is that you need 6 characters solution or won't work, this is because by default, zeroes on the left are deleted.
That's why it is useful to use control codes to identify when something is not working.