Default values for optional arguments

Optional arguments currently don't work with default values when the argument is `undefined` or `null`. This issue has also been observed by @artemgurzhii.

```js
// component.js
@argument(optional('string'))
foo = 'defaultValue'
```

```hbs
{{! parent-template.hbs}}

```

```hbs
{{! component-template.hbs}}
{{log foo}} {{! => undefined}}
```

Also, setting the value of the optional argument in the component during any point of the component lifecycle does not mutate the value:

```js
// component.js
didInsertElement () {
super.didInsertElement(...arguments)
this.set('foo', 'mutatedValue')
}
```

```hbs
{{! component-template.hbs}}
{{log foo}} {{! => undefined (still)}}
```

This is also true when argument values are supplied to the component:

```hbs
{{! parent-template.hbs}}

```

```js
// component.js
didInsertElement () {
super.didInsertElement(...arguments)
this.set('foo', 'mutatedValue')
}
```

```hbs
{{! component-template.hbs}}
{{log foo}} {{! => providedValue}}
```

If this is a bug, I assume it's a problem with the getter logic for optional arguments.