For dataobject type, provide a method telling whether object has empty parameter constructor?

Hi:

For current dataobject type, the minimum requirement is to provide json constructor
but there is no way for generator to know whether empty constructor is supported
It could be useful since for most dataobject, empty parameters constructor bring us convenience
If we generate all dataobject with empty constructor, the test will fail
since there is DataObjectWithOnlyJsonObjectConstructor and could not mapped to any constructor

Hope it helps