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.