QueryMapper does not support $function while converting _id

Suppose you have a document where _id is defined with a $function, like the following:

{
  "_id": {
    "$function": {
      "body": "some js code that takes no arguments"
      "args": []
      "lang": "js"
    }
  }
}

When invoking QueryMapper#getMappedObject (e.g. when building an aggregation pipeline), given that documentField.isIdField() is true, QueryMapper tries to convert every field of the subdocument to an ObjectId. Strings (that are not valid ObjectIds) are left untouched, but an empty array is converted to null, resulting in:

{
  "_id": {
    "$function": {
      "body": "some js code that takes no arguments"
      "args": null
      "lang": "js"
    }
  }
}

Sending this to mongodb will result in an exception since args is required and must be an array.