* 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).
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