Support Null type

[sebastien-rosset]

Not ready to submit as PR to upstream.

In OAS 3.1, the 'null' type is supported. This PR adds support for the 'null' type and partially provides a fix for OpenAPITools#5180. As an example, a schema may be defined as:

type: 'null'

This is useful in the following scenario when a property may be null or some referenced complex type:

Order:
  type: object
  properties:
    billTo:
      oneOf:
      - type: 'null'
      - $ref: "#/definitions/Address"

The python-experimental module performs a strict validation of the server response. If the response is null but the OAS schema does not allow null values, the client raises an exception such as the one below:

Exception when calling API: Invalid type for variable 'parent'.
Required value type is MoOptionalBaseMo and passed type was NoneType at
['received_data']['results'][0]['parent']

TODO:

1. Fail validation if openapi version is 3.0, AND one of the type is 'null' or one of the type is an array.

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