Ensure that disk.params is always defined (and a dict)

Commit cce46164 fixed upgrading from other 2.6 versions, but
accidentally broke upgrading from 2.5 (disk.params was left as None,
which breaks FillDict). The simplest way to handle params is to always
set them to an empty dict (disregarding what they currently contain).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>