`secret unlocker list --json` printed "unlockers": null for a vault with
no unlockers, and `secret vault list --json` printed "vaults": null when
vaults.d held no vaults. Both lists were nil slices, which encoding/json
writes as null. Each is now made as an empty slice before it is filled,
so an empty list prints as [].
vault.ListVaults therefore returns an empty slice instead of nil for an
empty vaults.d; its other callers only range over the result or check
its length.
Model: opus-5-5
