Python is not (e.g.) Haskell; Python strings are not lists whose contents happen to be characters.  Allowing an empty string here is a step backwards in the direction of "why not allow any string whose contents have an unambiguous meaning as bytes", i.e. the default encoding ASCII in Python 2.x.  Passing a string where bytes are expected is a programming error, and it should be rewarded with an exception, no matter if the string happens to be empty or not.