cmd/bundle: permit empty -prefix
cmd/bundle: permit empty -prefix

This enables bundling of programs with `main` method to single file.
I used this to upload code for the new codejam.

Change-Id: I9e65ba8717c1d81d6fb9bef8f234c21fc5b91421
GitHub-Last-Rev: 5d0d623c46ff1629e7371693ddf0916df328e319
GitHub-Pull-Request: golang/tools#32
Reviewed-on: https://go-review.googlesource.com/c/105515
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools