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.Core/PublishedCache/Internal/InternalPublishedContent.cs

@@ -20,6 +20,7 @@ namespace Umbraco.Cms.Core.PublishedCache.Internal
             Version = Guid.Empty;
             Path = string.Empty;
             ContentType = contentType;
+            Properties = Enumerable.Empty<IPublishedProperty>();
         }
 
         private Dictionary<string, PublishedCultureInfo>? _cultures;