do not use same abstraction for no object properties and empty

[hauner]

see #92

looks likes this case

components:
  schemas:
    AnyValue: {}

and this case

    Foo:
      type: object
      properties:
        bar:
          readOnly: true
          allOf:
            - $ref: '#/components/schemas/Bar'

should not both use NoDataType. For the second case the processor must not generate a class (see object-read-write-properties test case), for the first it could generate one. It should probably have an explicit EmptyDataType.