[in_app_purchase] Restore purchase on StoreKit not return a empty list when no purchases to restore

I read the changelog for 2.0.0:
```
Updates restorePurchases to emit an empty list of purchases on StoreKit when there are no purchases to restore (same as Android).
```

I expected an empty list in `purchaseStream`.

However, it still emit nothing when no purchases to restore on iOS.

It works fine on Android.