Fix unit tests

The call to ```DisposableTimer.DebugDuration()``` in ```ArchetypeValueConverter.ConvertDataToSource()``` expects the ```propertyType``` parameter to be not null, but the implementation of ```ArchetypeValueConverter.ConvertDataToSource()``` allows the ```propertyType``` parameter to be null. This breaks some of the unit tests.
I've added a null check in the call to ```DisposableTimer.DebugDuration()``` to counter this issue.