add a special case for Json/Bson optional fields

data/vibe/data/serialization.d

@@ -492,8 +492,14 @@ private template serializeValueImpl(Serializer, alias Policy) {
 
 						//skip optional attributes from serialization
 						if ((opt & OptionalDirection.out_) == OptionalDirection.out_) {
+							import vibe.data.json : Json;
+							import vibe.data.bson : Bson;
 							static if (isInstanceOf!(Nullable, typeof(vt))) {
 								if (vt.isNull) continue;
+							} else static if (is(typeof(vt) == Json)) {
+								if (vt.type == Json.Type.undefined || vt.type == Json.Type.null_ || (vt.type == Json.Type.object && !vt.length)) continue;
+							} else static if (is(typeof(vt) == Bson)) {
+								if (vt.type == Bson.Type.undefined || vt.type == Bson.Type.null_ || (vt.type == Bson.Type.object && !vt.length)) continue;
 							} else {
 								if (vt == typeof(vt).init) continue;
 							}
@@ -1707,3 +1713,23 @@ unittest {
 	auto deser = deserialize!(TestSerializer, Custom)(ser);
 	assert(deser.x == 42);
 }
+
+unittest {
+	import vibe.data.json;
+	struct Foo {
+		@optional
+		Json bar;
+	} 
+	Foo f;
+	assert(serializeToJsonString(f) == "{}");
+}
+
+unittest {
+	import vibe.data.bson;
+	struct Foo {
+		@optional
+		Bson bar;
+	} 
+	Foo f;
+	assert(serializeToJsonString(f) == "{}");
+}