fix: query wihout parent_id all result will come out

pkg/repository/comment.go

@@ -110,7 +110,12 @@ func (repo *CommentRepository) ListComments(ctx context.Context, req repository.
 	filter := bson.M{}
 	if req.ParentID != nil {
 		filter["parent_comment_id"] = req.ParentID
+	} else {
+		filter["parent_comment_id"] = bson.M{
+			"$in": []interface{}{"", nil},
 		}
+	}
+
 	if req.ReferenceID != nil {
 		filter["reference_id"] = req.ReferenceID
 	}