Refactor to introduce nullable types and reduce compiler warnings (#801)

* Introduce nullable types to Core, Infrastructure and PublicApi projects

* Refactor unit tests

* Fixed failing tests

* Introduce Parameter object for CatalogItem update method

* Refactor CataloItemDetails param object

* Refactor following PR comments

tests/IntegrationTests/Repositories/OrderRepositoryTests/GetByIdWithItemsAsync.cs

@@ -49,7 +49,7 @@ public class GetByIdWithItemsAsync
 
         //Act
         var spec = new OrderWithItemsByIdSpec(secondOrderId);
-        var orderFromRepo = await _orderRepository.GetBySpecAsync(spec);
+        var orderFromRepo = await _orderRepository.FirstOrDefaultAsync(spec);
 
         //Assert
         Assert.Equal(secondOrderId, orderFromRepo.Id);