Clean up build warnings

There are a number of build warnings that are thrown due to non-nullable types that are declared but not initialized, for example:

```
C:\git\bobs-used-bookstore-sample\app\Bookstore.Domain\Orders\OrderItem.cs(8,17): warning CS8618: Non-nullable property
'Order' must contain a non-null value when exiting constructor. Consider declaring the property as nullable. [C:\git\b
obs-used-bookstore-sample\app\Bookstore.Domain\Bookstore.Domain.csproj]
```

This is not a show stopper, but it is a bit sloppy and annoying.