fix partial filter expression

app/models/comment.js

@@ -44,7 +44,7 @@ CommentSchema.index({
   resource: 1,
   replyTo: 1,
 }, {
-  partialFilterExpression: { $exists: { replyTo: 1 } },
+  partialFilterExpression: { replyTo: { $exists: true } },
   name: 'comment_replies',
 });