Add a test for the ft_substr function when the second argument is larger than the number of characters given as the first argument.
If the second argument is greater than the number of characters given as the first argument, the ft_substr function returns an empty character.
Add a test for the ft_substr function when the second argument is larger than the number of characters given as the first argument. If the second argument is greater than the number of characters given as the first argument, the ft_substr function returns an empty character.