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.Web.BackOffice/Trees/LogViewerTreeController.cs

@@ -17,24 +17,28 @@ namespace Umbraco.Cms.Web.BackOffice.Trees
     [CoreTree]
     public class LogViewerTreeController : TreeController
     {
+        private readonly IMenuItemCollectionFactory _menuItemCollectionFactory;
+
         public LogViewerTreeController(
             ILocalizedTextService localizedTextService,
             UmbracoApiControllerTypeCollection umbracoApiControllerTypeCollection,
-            IEventAggregator eventAggregator)
+            IEventAggregator eventAggregator,
+            IMenuItemCollectionFactory menuItemCollectionFactory)
             : base(localizedTextService, umbracoApiControllerTypeCollection, eventAggregator)
         {
+            _menuItemCollectionFactory = menuItemCollectionFactory;
         }
 
-        protected override ActionResult<TreeNodeCollection?> GetTreeNodes(string id, FormCollection queryStrings)
+        protected override ActionResult<TreeNodeCollection> GetTreeNodes(string id, FormCollection queryStrings)
         {
             //We don't have any child nodes & only use the root node to load a custom UI
             return new TreeNodeCollection();
         }
 
-        protected override ActionResult<MenuItemCollection>? GetMenuForNode(string id, FormCollection queryStrings)
+        protected override ActionResult<MenuItemCollection> GetMenuForNode(string id, FormCollection queryStrings)
         {
             //We don't have any menu item options (such as create/delete/reload) & only use the root node to load a custom UI
-            return null;
+            return _menuItemCollectionFactory.Create();
         }
 
         /// <summary>