There needs to be a Optional[...] return type annotation for endpoints API

[jpkrajewski]

Description

Some endpoints can return data and empty response payload.

For example this PR #477 is introducing URL Monitoring endpoints, but get endpoint can return Datasequence and None which is breaking the api.

Traceback (most recent call last):
  File "/home/jakub/work/demo/snadbox.py", line 65, in <module>
    response = session.endpoints.url_monitoring.get_url_monitor()
  File "/home/jakub/work/catalystwan/catalystwan/endpoints/__init__.py", line 568, in wrapper
    return response.dataseq(self.return_spec.payload_type, self.resp_json_key)
  File "/home/jakub/work/catalystwan/catalystwan/response.py", line 198, in dataseq
    return DataSequence(cls, [cls.model_validate(item) for item in sequence])  # type: ignore
  File "/home/jakub/work/catalystwan/catalystwan/response.py", line 198, in <listcomp>
    return DataSequence(cls, [cls.model_validate(item) for item in sequence])  # type: ignore
  File "/home/jakub/.cache/pypoetry/virtualenvs/catalystwan-UKVG-1nx-py3.10/lib/python3.10/site-packages/pydantic/main.py", line 551, in model_validate
    return cls.__pydantic_validator__.validate_python(
pydantic_core._pydantic_core.ValidationError: 1 validation error for UrlMonitoringInfo
  Input should be a valid dictionary or instance of UrlMonitoringInfo [type=model_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.7/v/model_type

Replication script (URL monitoring have to be empty)

with create_manager_session(
    **sjlaas40cpu19,
    logger=logger,
) as session:
    session.endpoints.url_monitoring.add_url_monitor(UrlMonitoringConfig(
        url="/client/server/ready",
        threshold=10,
    ))
    response = session.endpoints.url_monitoring.get_url_monitor()

We can't add the Optional field for the endpoint because we will get error

Traceback (most recent call last):
  File "/home/jakub/work/demo/snadbox.py", line 6, in <module>
    from catalystwan.endpoints.url_monitoring import UrlMonitoring
  File "/home/jakub/work/catalystwan/catalystwan/endpoints/url_monitoring.py", line 12, in <module>
    class UrlMonitoring(APIEndpoints):
  File "/home/jakub/work/catalystwan/catalystwan/endpoints/url_monitoring.py", line 14, in UrlMonitoring
    def get_url_monitor(self) -> Optional[DataSequence[UrlMonitoringInfo]]:
  File "/home/jakub/work/catalystwan/catalystwan/endpoints/__init__.py", line 529, in __call__
    self.return_spec = self.specify_return_type()
  File "/home/jakub/work/catalystwan/catalystwan/endpoints/__init__.py", line 393, in specify_return_type
    raise APIEndpointError(f"Expected: {ReturnType} but return type {annotation}")
catalystwan.exceptions.APIEndpointError: Expected: typing.Union[NoneType, str, int, float, bool, typing.Dict[str, ForwardRef('JSON')], typing.List[ForwardRef('JSON')], bytes, dict, pydantic.main.BaseModel, catalystwan.typed_list.DataSequence[pydantic.main.BaseModel]] but return type typing.Optional[catalystwan.typed_list.DataSequence[catalystwan.models.url_monitoring.UrlMonitoringInfo]]