V10: fix more nullable references (#12321)

* Add Nullable and WarningsAsErrors to Build.props

* Remove Nullable from cs proj, add ImplicitUsings to build.props

* Fix errors in core

* Fix in JsonSchema

* Fix infrastructure

* Add non-ambiguous using

* Fix Web/Lucene

* Fix backoffice

* Fix up new build errors from merge

Co-authored-by: Nikolaj Geisle <niko737@edu.ucl.dk>

src/Umbraco.Web.Common/Extensions/ServiceCollectionExtensions.cs

@@ -176,7 +176,7 @@ namespace Umbraco.Extensions
         /// </remarks>
         public static TypeLoader AddTypeLoader(
             this IServiceCollection services,
-            Assembly entryAssembly,
+            Assembly? entryAssembly,
             ILoggerFactory loggerFactory,
             IConfiguration configuration)
         {