Invalid Code is generated for lists inside a union

[diesieben07]

Describe the bug
When a list occurs inside a union, this call causes the list template to initialize the value with UNSET instead of []. The list template then tries to call append on UNSET, which of course does not work.

OpenAPI Spec File
A link to your openapi.json which produces this issue.

Desktop (please complete the following information):

• OS: Ubuntu 22.04.2
• Python Version: 2.10.8
• openapi-python-client version 0.14.0

Additional context
The above OpneAPI spec file generates the following code, for reference:

def _parse_response(
    *, client: Client, response: httpx.Response
) -> Optional[Union["GetExampleResponse200Type0", List["GetExampleResponse200Type1Item"]]]:
    if response.status_code == HTTPStatus.OK:

        def _parse_response_200(
            data: object,
        ) -> Union["GetExampleResponse200Type0", List["GetExampleResponse200Type1Item"]]:
            try:
                if not isinstance(data, dict):
                    raise TypeError()
                response_200_type_0 = GetExampleResponse200Type0.from_dict(data)

                return response_200_type_0
            except:  # noqa: E722
                pass
            if not isinstance(data, list):
                raise TypeError()
            response_200_type_1 = UNSET # PROBLEM IS HERE
            _response_200_type_1 = data
            for response_200_type_1_item_data in _response_200_type_1:
                response_200_type_1_item = GetExampleResponse200Type1Item.from_dict(response_200_type_1_item_data)

                response_200_type_1.append(response_200_type_1_item)

            return response_200_type_1

        response_200 = _parse_response_200(response.json())

        return response_200
    if client.raise_on_unexpected_status:
        raise errors.UnexpectedStatus(response.status_code, response.content)
    else:
        return None

[kgutwin]

It looks like I also identified this in #928 and there is a proposed fix in #929.