Pull request: 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)