Reproduction for meteor/meteor#6051.
git clone https://github.com/hwillson/meteor-issue-6051.git
cd meteor-issue-6051; meteor npm i; meteor
When widgetsCollection.insert({ _id: 'from-client', name: undefined });
is
called from the client, a { "_id" : "from-client" }
record is created in
Mongo. When widgetsCollection.insert({ _id: 'from-server', name: undefined });
is called from the server, a { "_id" : "from-server", "name" : null }
record
is created in Mongo.