Few more NRT tweaks (#12323)

* Amended GetAll() on IDataTypeService to return an empty collection rather than null.

* Added a ClearSessionValue method to ISessionManager (given you can no longer set a value to null).

* Allow for null values in a StatefulNotification.

* Removed obsoletion of synchronous messages on TreeControllerBase.

* Fixed further CS8620 warnings in core project.

* Further fix to nullable warning.

* Aligned nullablility of retreiving tree nodes and menus, synchronously or asynchronously (such that we no longer can get null values, always empty collection objects).

src/Umbraco.Infrastructure/Routing/RedirectTrackingHandler.cs

@@ -90,7 +90,7 @@ namespace Umbraco.Cms.Core.Routing
                 notification.State[NotificationStateKey] = new OldRoutesDictionary();
             }
 
-            return (OldRoutesDictionary)notification.State[NotificationStateKey];
+            return (OldRoutesDictionary?)notification.State[NotificationStateKey] ?? new OldRoutesDictionary();
         }
 
         private void StoreOldRoute(IContent entity, OldRoutesDictionary oldRoutes)