Whenever mongosh tries to insert undefined values, those undefined values get converted to null in the driver logic. This effectively ensures that users of mongosh are not able to insert any values of undefined type into MongoDB. This is not true in the legacy mongo shell, as the legacy shell allows you to insert values of undefined type. 

Instead of this implicit conversion from undefined -> null on the driver side, we should perform that conversion on the server side, to effectively disallow any insertion of new undefined values into MDB collections.