guix-package: When rolling back to nothingness, point to the empty profile.

guix-package: When rolling back to nothingness, point to the empty profile.

Suggested by Andreas Enge <andreas@enge.fr> at
<http://lists.gnu.org/archive/html/bug-guix/2013-01/msg00316.html>.

* guix-package.in (roll-back): Check whether PROFILE is valid using
  `file-exists?'.  When NUMBER is zero, just emit a notice.  When
  PREVIOUS-NUMBER is zero and PREVIOUS-PROFILE does not exist, build the
  empty profile, and link to it.
* tests/guix-package.sh: Add tests.
* doc/guix.texi (Invoking guix-package): Document the new behavior.