[BUG] [PYTHON] Malformed type hints on 6.3.0-SNAPSHOT with 303 status response

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Clients are generated with syntax errors using the spec provided.

Sample of code with syntax errors:

class ThingDownload(BaseApi):
    # this class is used by api classes that refer to endpoints with operationId fn names

    @typing.overload
    def thing_download(
        self,
        path_params: RequestPathParams = frozendict.frozendict(),
        accept_content_types: typing.Tuple[str] = _all_accept_content_types,
        stream: bool = False,
        timeout: typing.Optional[typing.Union[int, typing.Tuple]] = None,
        skip_deserialization: typing_extensions.Literal[False] = ...,
    ) -> typing.Union[
    ]: ...

It is not valid for this typing.Union to be empty, as shown at import time:

E       ) -> typing.Union[
E                        ^
E   SyntaxError: expected ':'

openapi-generator version

openapi-generator-cli 6.3.0-SNAPSHOT
  commit : 188c39d
  built  : 2022-11-11T16:58:19-05:00
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

OpenAPI declaration file content or url

Gist: https://gist.github.com/syntaxaire/9e584c63ac4ce71c7eda882a4d6837dc

Generation Details

java -jar openapi-generator-cli.jar generate --global-property skipFormModel=false -i empty-union.json -g python -o empty-union

Steps to reproduce

• Download the spec from the provided gist.
• Use the latest git version with the command line above to generate the client
• Install the client and run its unit tests, the above syntax error will be shown when imported

Start with the empty-union.json specification from the linked gist and reproduce the issue using the documented openapi-generator CLI command with the Python generator. Inspect the generated client's type hints, then run its unit tests; done means the client imports and tests run without the reported SyntaxError.