Support None / null as enum values #504

Describe the bug
Similar to #357, partially fixed in #500.
Client generation fails if an enum like below is present in the generated OpenAPI schema.

NullEnum:
  enum:
  - null

The context here is that we have a Django model with char / text fields that have null=True, blank=True set. We use Django Rest Framework / drf-spectacular to generate an OpenAPI schema from our models and REST API endpoints. This generated schema automatically includes logic like below:

ModelName:
  type: object
  description: ModelName serializer
  properties:
    field_name:
      nullable: true
      oneOf:
      - $ref: '#/components/schemas/FieldNameEnum'
      - $ref: '#/components/schemas/BlankEnum'
      - $ref: '#/components/schemas/NullEnum'

FieldNameEnum, BlankEnum, and NullEnum are like below:

FieldNameEnum:
  enum:
  - stringvalue1
  - stringvalue2
  type: string
BlankEnum:
  enum:
  - ''
NullEnum:
  enum:
  - null

NullEnum accepts a literal null / None, not a string "null" (it's not a string enum).

To Reproduce
Steps to reproduce the behavior:

1. Make a new OpenAPI Spec File matching the one below.
2. Run "openapi-python-client generate --path your_spec_file.yaml"
3. See an error that null / None is not allowed.

Expected behavior
API client generation succeeds. An enum with None / null as an option is valid based on #500 (comment), so we would like to see this supported in openapi-python-client if easily possible.

In version 0.10.4, we were able to generate the clients successfully by changing the enum_property.py file. If value was None / not a string, it would skip calling remove_string_escapes(value) and just store None directly into output[sanitized_key].

OpenAPI Spec File

openapi: 3.0.3
info:
  title: title
  version: 1.0.0
paths:
  /path/to/something/:
    get:
      responses:
        '200':
          description: ''
components:
  schemas:
    ValueEnum:
      enum:
      - value
    NullEnum:
      enum:
      - null

Desktop (please complete the following information):

OS: Linux 4.18
Python Version: 3.6.8
openapi-python-client version 0.10.5

Additional context
Error(s) encountered while generating, client was not created

Failed to parse OpenAPI document

3 validation errors for OpenAPI
components -> schemas -> NullEnum -> $ref
field required (type=value_error.missing)
components -> schemas -> NullEnum -> enum -> 0
none is not an allowed value (type=type_error.none.not_allowed)
components -> schemas -> NullEnum -> enum -> 0
none is not an allowed value (type=type_error.none.not_allowed)