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.Core/Notifications/NotificationExtensions.cs

@@ -4,7 +4,7 @@ namespace Umbraco.Cms.Core.Notifications
 {
     public static class NotificationExtensions
     {
-        public static T WithState<T>(this T notification, IDictionary<string, object>? state) where T : IStatefulNotification
+        public static T WithState<T>(this T notification, IDictionary<string, object?>? state) where T : IStatefulNotification
         {
             notification.State = state!;
             return notification;