Check for empty string case.

[atgreen]

After updating quicklisp and sbcl recently, I've discovered that I need to add this bit of code. The empty string will cause a subseq error later in the function ... essentially (subseq "" 1)

I don't know if this is because snooze changed, hunchtoot changed, sbcl changed, or ??? But it's a reasonable patch, IMO.

[mdbergmann]

I also came across this issue wondering what suddenly caused it.