[codegen][validation] Add support for 'null' type

Created by: sebastien-rosset

Add support for the 'null' type, and retain existing behavior for OAS 3.0.x documents that don't use the 'null' type.

In OAS 3.1, the 'null' type is supported, and some existing 3.0.x documents use the 'null' type, even though it's not actually supported by the OAS 3.0 spec.

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"

In 3.0.x, the above snippet could be written differently by using "nullable: true", but the nullable attribute in being deprecated in OAS 3.1. See #5180 for issues related to the 'nullable' attribute.

When a OAS document does not explicitly specify a 'null' value, the generated code should not accept a null value. In practice, some language generators such as go-experimental are lenient and allow null values. Other generators such as the python-experimental module perform a strict validation of the payload (e.g. 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']

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