default value on fragmentArray not working

Hello,

I'm running into an issue where an attribute on my model that is set to a `fragmentArray` is always set to `null` by default rather than an empty array (default according to docs). I've tried setting in multiple ways, with no luck:

```js
customRates: fragmentArray('rate', { defaultValue: function() {
return [];
}}),
```

```js
customRates: fragmentArray('rate', { defaultValue: [] })
```

Here's the `rate` fragment I'm referencing:

```js
// rate.js
import attr from 'ember-data/attr';
import Fragment from 'model-fragments/fragment';
import { validator, buildValidations } from 'ember-cp-validations';

const Validations = buildValidations({
type: validator('presence', true),
amount: [
validator('presence', true),
validator('number', {
allowString: true,
gt: 0,
lte: 100
})
],
serviceType: validator('presence', true),
serviceId: validator('presence', true)
});

export default Fragment.extend(Validations, {
type: attr('string'),
amount: attr('number'),
serviceType: attr(),
serviceId: attr('number')
});
```

I'm running ED 2.11 and EDMF 2.11.

Thanks in advance!

Start with the model declaration using fragmentArray and the rate.js fragment shown in the report, then trace how defaultValue is handled for fragment arrays. Reproduce the ED 2.11 and EDMF 2.11 behavior and verify that an unset customRates attribute becomes an empty array rather than null.

索引モデルが issue の本文から書いたものです。