[Python-experimental] JSON schema 'null' type should be modeled as 'none_type'

Created by: sebastien-rosset

It looks like recently something changed because the JSON schema null type is now causing problems. I'm not sure what specific commit caused this problem. The getTypeString function in PythonClientExperimentalCodegen.java is incorrectly returning null when the input Schema is the Null type.

For example, with the following schema, the generated python-experimental code has syntax errors.

Foo:
  oneOf:
    - type: 'null'
    - '#/components/schemas/Bar'

Without this PR, the following import is incorrectly generated:

try:
    from openapi_client.models import null
except ImportError:
    null = sys.modules[
        'openapi_client.models.null']

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