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_list_test.go

@@ -1,13 +1,14 @@
 // Unlocker List Tests
 //
-// Tests for `secret unlocker list` behavior when an unlocker's metadata
-// cannot be read or used:
+// Tests for `secret unlocker list`:
 //
 //   - TestUnlockersListToleratesCorruptMetadata: one unlocker's corrupt
 //     metadata does not stop the others from being listed.
 //   - TestUnlockersListSkipsUnreadableMetadata: an unlocker whose metadata
 //     file cannot be checked for or read is left out, and the other is
 //     still listed.
+//   - TestUnlockersListEmptyJSON: a vault with no unlockers lists them as
+//     an empty JSON list.
 
 //nolint:testpackage // white-box test of unexported internals
 package cli
@@ -321,3 +322,36 @@ func TestUnlockersListSkipsUnreadableMetadata(t *testing.T) {
 		})
 	}
 }
+
+// TestUnlockersListEmptyJSON is a regression test for
+// https://git.eeqj.de/sneak/secret/issues/148: listing the unlockers of a
+// vault that has none, as after its last unlocker is removed, printed
+// "unlockers": null in JSON form instead of an empty list.
+func TestUnlockersListEmptyJSON(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	vaultDir := filepath.Join(listTestStateDir, "vaults.d", listTestVaultName)
+
+	require.NoError(t, afero.WriteFile(
+		fs, filepath.Join(listTestStateDir, "currentvault"),
+		[]byte(listTestVaultName), listTestFilePerm,
+	))
+	require.NoError(t, fs.MkdirAll(
+		filepath.Join(vaultDir, listTestUnlockersDirName), listTestDirPerm,
+	))
+
+	var buf bytes.Buffer
+
+	cmd := &cobra.Command{}
+	cmd.SetOut(&buf)
+
+	instance := &Instance{fs: fs, stateDir: listTestStateDir, cmd: cmd}
+	require.NoError(t, instance.UnlockersList(true))
+
+	var decoded map[string]any
+
+	require.NoError(t, json.Unmarshal(buf.Bytes(), &decoded))
+	assert.Equal(t, []any{}, decoded["unlockers"],
+		"no unlockers must be printed as [], not %s", buf.String())
+}