Was just thinking it could return true or false even in a null case.

It doesn't make any sense without any context. A string neither does nor does not end with no-one-knows-what. Returning a value is just hiding an error that someone (maybe you) will have to find one day when something weird happens because of it.

Null ending parameter is (gracefully) handled

?
It's handled the same way as a null str.

It is handled: an exception is thrown. How else would you handle that?

@dinglemouse: Thanks for your explanations and comments.

On the contrary, this Kata is very precisely worded:

Your task is to return the first longest string consisting of k consecutive strings taken in the array.

• You are asked to look for consecutive strings (how many of the input strings to concatenate determined by k)
• From those candidate concatenated strings you need to find the longest
• And if there happen to be several candidate concatenated strings having the same longest length then you need to return the first one

One can't say it simpler:

Your task is to return the first longest string consisting of k consecutive strings taken in the array.

In other words construct all strings constitued of k consecutive (following one another) strings of the array and return the first which has the longest length.