[racket] Racket 5.1.3 bug running exes from empty path

Hi there,

when raco exe is used to create a standalone .exe and I try to run the  
resulting exe from a DOS box with an empty path, the runtime support  
terminates with the message

with-input-from-file: expects type <path or string> as 1st argument,  
given: #f;
other arguments were: #<procedure>

  === context ===
embedded-load

(This is Windows XP).

To reproduce:

- run raco exe on some project
- start a DOS box in that directory
- run
   set path=
   in that DOS box
- try to run the executable

Setting the path to just something (even a non existing entry) solves  
the problem.

It's probably not a big deal because most likely, everybody will have  
SOMEthing in the path. Just thought I'd let you know so you won't  
focus all your energy on easter egg regress tests (then again, easter  
is coming up fast! ;-))