Presence validation on belongsTo relationship stopped working on 3.11.

### Environment

- Ember Version: 3.11.1
- Ember CLI Version: 3.11.0
- Ember CP Validations Version: 4.0.0-beta.9

### Steps to Reproduce
After I upgraded from ember 3.10 to 3.11 the presence validations on belongsTo relationships stopped working. Looks like it does not detect that the associated record has changed and displays `This field can't be blank` even when record is well assigned.

After I ran `notifyPropertyChange` on that property the validations get updated and works correctly.

This is how my model looks like:
```javascript
import attr from 'ember-data/attr';
import Model from 'ember-data/model';
import { belongsTo } from 'ember-data/relationships';
import { buildValidations, validator } from 'ember-cp-validations';

const Validations = buildValidations({
title: [
validator('presence', true)
],
author: [
validator('presence', true)
]
});

export default Model.extend(Validations, {
title: attr('string'),

author: belongsTo()
});
```

Start from the shown JavaScript model and reproduce the belongsTo presence-validation case on Ember 3.11.1 with ember-cp-validations 4.0.0-beta.9, comparing it with Ember 3.10. Trace how assignment updates validation state versus the manual notifyPropertyChange path. Done means an assigned author no longer shows “This field can't be blank” without manual notification.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.