Paper Select 'required' behaviour doesn't work when 'selected' is set to an empty promise

A `{{paper-select}}` component in a `{{paper-form}}` and bound to a `belongsTo` relationship does not behave correctly when `required=true`, if the relationship is empty and the `selected` property is set to the related model. For example, if `selected=model.otherModel` when there is no associated `otherModel`. The reason seems to be that when there isn't a related model, `model.otherModel` is equal to an Ember Data PromiseObject whose `content` is `null`, instead of `model.otherModel` actually being `null` itself.

The result is that the `*` to indicate that the field is required doesn't appear, and the form passes validation even when no value is selected. The correct behaviour can be obtained by setting `selected=(if model.otherModel.id model.otherModel)` so that `selected` is `null` when the other model is missing.

I've created a [demo repo](https://github.com/mikeu/paper-select-related-null) that reproduces the issue.