Allow Nullable AutoIncrementId

Tried to re-use a Service DTO for a quick OrmLite demo today with a colleague.  By chance, the DTO had `public int? Id { get; set; }`  that I applied `[AutoIncrement]` to.  Was throwing `NullReferenceException` due to `default(T)` of `int?` being `null`.

I realise it's not a typical use-case to have `int?` as a key and that it's better to have a separate objects for the ORM, but here's a quick fix and tests anyway.