storaged: Handle null slots
storaged: Handle null slots

The previous checks in CryptoKeyslots don't completely rule out that
slots === null, which is a potential crash. The code below does error
handling based on "keys is empty", so set it to an empty list on a null
slots value.

Spotted by Coverity.

Fixes #15988

cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit