if you have something like this in .htaccess
ErrorDocument 404
http://www.yourdmain.com/404page.html
then you don't need to change it.
When you upload blank robots.txt to your domain root you should be able open
http://yourdomain.com/robots.txt in bowser and get a blank page. If that's not the case you are probably redirecting somewhere.
Maybe it's best to post contents of your .htaccess just to be sure something is not hiding in there.