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/Web/ISessionManager.cs

@@ -2,7 +2,10 @@ namespace Umbraco.Cms.Core.Web
 {
     public interface ISessionManager
     {
-        string? GetSessionValue(string sessionName);
-        void SetSessionValue(string sessionName, string value);
+        string? GetSessionValue(string key);
+
+        void SetSessionValue(string key, string value);
+
+        void ClearSessionValue(string key);
     }
 }