> Is there somewhere some info about "" being NIL?
• to represent a string of length zero
Besides for standard normal form, NIL is also recognized as (),  or ""
> I'd like to know why, maybe with some historical hindsight,
There is no special history to it.
> The language I'm trying to embed in PicoLisp would love to have
> "" and NIL different !
Why? I believe this is a wonderful feature! A lot of PicoLisp's
expressive power is a result of this universality of NIL.