You're right, it shouldn't send the headers until a non-empty string
occurs.  I don't see any problem with treating it as a bug, and fixing it.
Your patch will also allow non-compliant behavior, though.  It seems to me
it would be better to fix the logic in write() to not call send_headers()
if len(data)==0.  That way, it will still error with "write() before
start_response()" in the non-compliant case, but fix the compliance error.
Feel free to reopen/retitle this issue for that.