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/Services/MemberService.cs

@@ -852,7 +852,7 @@ namespace Umbraco.Cms.Core.Services
             }
         }
 
-        private void DeleteLocked(ICoreScope scope, IMember member, EventMessages evtMsgs, IDictionary<string, object>? notificationState = null)
+        private void DeleteLocked(ICoreScope scope, IMember member, EventMessages evtMsgs, IDictionary<string, object?>? notificationState = null)
         {
             // a member has no descendants
             _memberRepository.Delete(member);