Always genenerate module DataRoot

The concept of each module having a root is (i.e. a namespace) is quite prevalent in YANG. We currently skip generation of DataRoot if there are no instantiated data schema nodes present.

This has an unfortunate effect on the run-time view of things, as we really want each module to have a RuntimeType – but if there is no DataRoot, there is no GeneratedType to fulfill the requirement to fulfil the API requirements of ModuleRuntimeType.

Eliminate this special case and always generate a DataRoot class.