What black magic is this?

I had a strange issue where my code would pass the test, but fail the submit with the random tests. Sometimes it would not fail. I think the issue had to do with my initial conditional checking if the string was empty and index was not 0 or greater than the array length. But I tried a lot of different combinations, and it still failed sometimes and didn't others. Now that I see the rest of the answers, I don't see why mine would have failed at all. Does anyone have insight on this?

I overcomplicated this one. This is a clean answer, thank you.