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/FileSystemTreeController.cs

@@ -49,7 +49,7 @@ namespace Umbraco.Cms.Web.BackOffice.Trees
             treeNode.AdditionalData["jsClickCallback"] = "javascript:void(0);";
         }
 
-        protected override ActionResult<TreeNodeCollection?> GetTreeNodes(string id, FormCollection queryStrings)
+        protected override ActionResult<TreeNodeCollection> GetTreeNodes(string id, FormCollection queryStrings)
         {
             var path = string.IsNullOrEmpty(id) == false && id != Constants.System.RootString
                 ? WebUtility.UrlDecode(id).TrimStart("/")