Fix for U4-5698 - EntityService.GetObjectType throws for deleted Id's

For example

    var test = ApplicationContext.Current.Services.EntityService.GetObjectType(3333333);

This fix works but I suspect it has wider implications and there is probably a better method of handling it as I doubt we want to be modifying PetaPoco.cs, maybe even try/catch in the GetObjectType method, over to @Shandem  and @sitereactor....?