cherry-pick: aghalg: impl json.Marshaler for NullBool

Updates #4735.

Squashed commit of the following:

commit 93a0b1dc6b668f7d9fd89d06b8f0f24dcd345356
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Jul 11 17:02:36 2022 +0300

    aghalg: impl json.Marshaler for NullBool

internal/aghalg/nullbool.go

@@ -40,6 +40,14 @@ func BoolToNullBool(cond bool) (nb NullBool) {
 	return NBFalse
 }
 
+// type check
+var _ json.Marshaler = NBNull
+
+// MarshalJSON implements the json.Marshaler interface for NullBool.
+func (nb NullBool) MarshalJSON() (b []byte, err error) {
+	return []byte(nb.String()), nil
+}
+
 // type check
 var _ json.Unmarshaler = (*NullBool)(nil)
 

internal/aghalg/nullbool_test.go

@@ -10,6 +10,52 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestNullBool_MarshalJSON(t *testing.T) {
+	testCases := []struct {
+		name       string
+		wantErrMsg string
+		want       []byte
+		in         aghalg.NullBool
+	}{{
+		name:       "null",
+		wantErrMsg: "",
+		want:       []byte("null"),
+		in:         aghalg.NBNull,
+	}, {
+		name:       "true",
+		wantErrMsg: "",
+		want:       []byte("true"),
+		in:         aghalg.NBTrue,
+	}, {
+		name:       "false",
+		wantErrMsg: "",
+		want:       []byte("false"),
+		in:         aghalg.NBFalse,
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := tc.in.MarshalJSON()
+			testutil.AssertErrorMsg(t, tc.wantErrMsg, err)
+
+			assert.Equal(t, tc.want, got)
+		})
+	}
+
+	t.Run("json", func(t *testing.T) {
+		in := &struct {
+			A aghalg.NullBool
+		}{
+			A: aghalg.NBTrue,
+		}
+
+		got, err := json.Marshal(in)
+		require.NoError(t, err)
+
+		assert.Equal(t, []byte(`{"A":true}`), got)
+	})
+}
+
 func TestNullBool_UnmarshalJSON(t *testing.T) {
 	testCases := []struct {
 		name       string