fix: add support for null response fields

[eshanholtz]

This PR adds support for receiving null fields in the response json. Also fixes enum naming conflict.

[thinkingserious]

I verified these changes no longer cause an exception when generating a python client with the following steps:

git clone git@github.com:twilio/twilio-oai.git
cd twilio-oai
git fetch --all
git checkout origin/nullable
git pull origin nullable
npm install @openapitools/openapi-generator-cli -g
npx @openapitools/openapi-generator-cli generate -i ~/Projects/code-reviews/1183/twilio-oai/spec/yaml/twilio_api_v2010.yaml -g python -o ./tmp/test/
cd tmp/test
pip install python-dateutil
python setup.py install

I then created a test script that used to cause an exception and verified that the exception is no longer thrown:

import openapi_client
import os
from openapi_client.api import default_api
from pprint import pprint

configuration = openapi_client.Configuration(
    host = "http://api.twilio.com"
)

configuration = openapi_client.Configuration(
    username = os.environ['TWILIO_ACCOUNT_SID'],
    password = os.environ['TWILIO_AUTH_TOKEN']
)

with openapi_client.ApiClient(configuration) as api_client:
    api_instance = default_api.DefaultApi(api_client)
    account_sid = os.environ['TWILIO_ACCOUNT_SID']
    to = os.environ['TWILIO_TO_PHONE_NUMBER']
    body = "Hello World!"
    _from = os.environ['TWILIO_FROM_PHONE_NUMBER']

    try:
        api_response = api_instance.create_message(account_sid, to, _from=_from, body=body)
        pprint(api_response)
    except openapi_client.ApiException as e:
        print("Exception when calling DefaultApi->create_message: %s\n" % e)

[shwetha-manvinkurke]

Have you tried regenerating the twilio-go from the new specs?

[eshanholtz]

@shwetha-manvinkurke yes, the only thing it changes is that properties in the response object struct are now pointers, so they can have a null value. golang handles pointer/non-pointer values by design.