Well, if the function was designed by me, then the completely obvious answers are '' and a NullPointerException. Anything else is bug-prone — if the first case does something different, then nearly every caller of the function will need to check to see if the count is zero, or it will get behavior for that case that is not correct for its purposes; and if the second case does something different, then the function is choosing to provide incorrect output instead of crashing.

Also, I would have called it something different.

However, over the years, I've learned that many things that are obvious to me are not, in fact, true. So, what are the arguments for any other possible behavior?

Backward compatibility could be one such factor.

In any case, documenting edge conditions helps in code maintenance.

Since I'm coming from Python, JS, and C, passing a null instead of a string doesn't seem like an "edge condition"; it's the same kind of error as passing an integer or Map instead of a string. Passing 0, I suppose, is an edge condition.