Allow responses with no content

[acgray]

Empty API responses are allowed by the OpenAPI spec but currently cause an error when parsing a schema which uses them:

https://github.com/triaxtec/openapi-python-client/blob/master/openapi_python_client/openapi_parser/responses.py#L82-L83

Instead, they should be supported, probably returning None as the response.

[dbanty]

Thanks @acgray , will release this in 0.4.2 shortly.