Print an empty unlocker or vault list as [] in JSON (closes #148)

`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

internal/cli/unlockers.go

@@ -314,7 +314,8 @@ func (cli *Instance) UnlockersList(jsonOutput bool) error {
 		return err
 	}
 
-	var unlockers []UnlockerInfo
+	// Not nil, so that a vault with no unlockers prints [] in JSON, not null
+	unlockers := make([]UnlockerInfo, 0, len(unlockerMetadata))
 
 	for _, unlockerID := range slices.Sorted(maps.Keys(unlockerMetadata)) {
 		metadata := unlockerMetadata[unlockerID]