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/FunctionalTests/Web/Pages/HomePageOnGet.cs

@@ -1,6 +1,4 @@
-using System.Net.Http;
-using System.Threading.Tasks;
-using Microsoft.eShopWeb.FunctionalTests.Web;
+using Microsoft.eShopWeb.FunctionalTests.Web;
 using Xunit;
 
 namespace Microsoft.eShopWeb.FunctionalTests.WebRazorPages;