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/vault/management.go

@@ -140,8 +140,9 @@ func ListVaults(fs afero.Fs, stateDir string) ([]string, error) {
 		return nil, fmt.Errorf("failed to read vaults directory: %w", err)
 	}
 
-	// Extract vault names
-	var vaults []string
+	// Extract vault names. Not nil, so that no vaults prints [] in JSON,
+	// not null.
+	vaults := make([]string, 0, len(entries))
 
 	for _, entry := range entries {
 		if entry.IsDir() {