@ember-data/record-data RecordData.isEmpty implementation should consider isNew in calculation

[runspired]

The implementation is here: Source

 // this is a hack bc we don't have access to the state machine
  //   and relationships need this info and @runspired didn't see
  //   how to get it just yet from storeWrapper.
  isEmpty() {
    return this.__attributes === null && this.__inFlightAttributes === null && this.__data === null;
  }

This leads to a bug whereby store.createRecord('user') (note the lack of any second arg) can incorrectly trigger fetches for relationships with the new user record because the internals consider empty records to be fetchable. It also means that in more recent ember-data versions records begin their life isEmpty if created in this way, as we now pull this state from the RecordData instance.

[runspired]

cc @kategengler