fs: fix tristate conversion to JSON

fs/tristate_test.go

@@ -85,3 +85,18 @@ func TestTristateUnmarshalJSON(t *testing.T) {
 		assert.Equal(t, test.want, got, test.in)
 	}
 }
+
+func TestTristateMarshalJSON(t *testing.T) {
+	for _, test := range []struct {
+		in   Tristate
+		want string
+	}{
+		{Tristate{}, `null`},
+		{Tristate{Valid: true, Value: true}, `true`},
+		{Tristate{Valid: true, Value: false}, `false`},
+	} {
+		got, err := json.Marshal(&test.in)
+		require.NoError(t, err)
+		assert.Equal(t, test.want, string(got), fmt.Sprintf("%#v", test.in))
+	}
+}