Indexing of NULL in mongoDB [duplicate]

I have a mongodb collection like

{
    template_id: { type: String, required: true },
    owner: {
        user_id: { type: String, required: true }
    },
    assignee: {
        user_id: { type: String, required: false }
    }
    //...other fields
}

There is a compound index (non-sparse & non-unique) for this document,

{ template_id: 1, owner: 1, assignee: 1 }

In this document the assignee may have value or may be null. However, the query pattern for fetching records having null is important.

Are records having null values indexed so that I can use for querying based in them?