The documentation of Windows.Services.Store.StoreContext.GetAppAndOptionalStorePackageUpdatesAsync is incomplete.
The method is succeeding and returning a null value when there is no internet connection.
The documentation should be updated to something like:
-returns
An asynchronous operation that, on successful completion, returns a collection of StorePackageUpdate objects that represent the packages that have updates available or null if there is not internet connection.
Another (and probably better) option is to fix the implementation to return an empty list instead of a null value.