I've asked about this in the chat but haven't heard anything regarding if it's ever valid to access array[0]. Presuming the answer is no:

Since fish array indices start at 1 (unlike many other programming/scripting languages), I'm probably not the only one that forgets and starts at 0 from time to time. If [0] has no special meaning, it should be a hard error (instead of returning an empty result) so that it is clear there is something wrong with the script.