Undefined and an empty list both have a length of -1, so for what you need to use it for <bold>THEY ARE THE SAME THING</bold>. If you want you can return (""), but then you will have a single element array that contains nothing.
Comment on (c-era)Re: Empty list as a return value
Well, they both have a length of 0. Perhaps you are confusing the length with what $#ARRAYTHINGY returns, which is one less than the length, but therefore suitable for use with upper index bounds.