libweave: Fixed error message

auth_code is optional and can be empty.

BUG=none
TEST=none

Change-Id: Ic20598bd6e48cf3b1fbf31ac2af03c7cce908167
Reviewed-on: https://chromium-review.googlesource.com/289694
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>