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/vault_list_test.go

@@ -0,0 +1,42 @@
+package cli_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/spf13/cobra"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/secret/internal/cli"
+	"sneak.berlin/go/secret/internal/secret"
+)
+
+// TestVaultListEmptyJSON is a regression test for
+// https://git.eeqj.de/sneak/secret/issues/148: listing the vaults when
+// vaults.d holds none printed "vaults": null in JSON form instead of an
+// empty list.
+func TestVaultListEmptyJSON(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, fs.MkdirAll(
+		filepath.Join(testStateDir, "vaults.d"), secret.DirPerms,
+	))
+
+	var buf bytes.Buffer
+
+	cmd := &cobra.Command{}
+	cmd.SetOut(&buf)
+
+	c := cli.NewCLIInstanceWithStateDir(fs, testStateDir)
+	require.NoError(t, c.ListVaults(cmd, true))
+
+	var decoded map[string]any
+
+	require.NoError(t, json.Unmarshal(buf.Bytes(), &decoded))
+	assert.Equal(t, []any{}, decoded["vaults"],
+		"no vaults must be printed as [], not %s", buf.String())
+}