The issue here is that null in MongoDB is treated as its own distinct type, which is quite different than how most developers would think about a nullable field in statically defined languages. A conversion feature/project is on deck for this quarter so hopefully we will get this sorted for you soon. In the meantime, you can use the workaround Sumedha described: define an empty value, or change the field to be required. Another option is leveraging our new Mixed type here:
Hope this helps