_NULL_TYPE not JSON serializable

[DrWacker]

When Using the Nullable type option if the value isn't set and is therefore _NULL_TYPE json.dump will throw an error. See code example below.

>>> from dataclasses_jsonschema.type_defs import _NULL_TYPE
>>> my_dict = { "foo" : _NULL_TYPE }
>>> import json
>>> my_str = json.dumps(my_dict)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>                                                                                                                    File "/usr/lib/python3.7/json/__init__.py", line 231, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python3.7/json/encoder.py", line 199, in encode                                                                                           chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python3.7/json/encoder.py", line 257, in iterencode                                                                                       return _iterencode(o, 0)
  File "/usr/lib/python3.7/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type type is not JSON serializable
>>>