code generated for `date-time` fromat can not parse null response data

先定位為具有 date-time 格式的 OpenAPI 欄位產生模型反序列化的產生器邏輯，然後使用提供的 nullable updateTime schema 和回應資料重現該問題。為 null date-time 值加入回歸測試，並驗證產生的用戶端能夠接受該回應而不會引發 TypeError。

Describe the bug
code generated for date-time fromat in models file like this for now:

_update_time = d.pop("updateTime", UNSET)
update_time: Union[Unset, datetime.datetime]
if isinstance(_update_time, Unset):
    update_time = UNSET
else:
    update_time = isoparse(_update_time)

if response data of updateTime field is null, the _update_time variable will be set to None, so isoparse function will rasie TypeError except.

self = <dateutil.parser.isoparser.isoparser object at 0x7f94bf7ae950>, dt_str = None

    def _parse_isodate_common(self, dt_str):
>       len_str = len(dt_str)
E       TypeError: object of type 'NoneType' has no len()

../../../../.local/lib/python3.10/site-packages/dateutil/parser/isoparser.py:213: TypeError

OpenAPI Spec File

{
    "updateTime": {
        "type": "string",
        "description": "updateTime",
        "format": "date-time"
    }
}

Desktop (please complete the following information):

• OS: windows wsl ubuntu 22.0
• Python Version: 3.10.0
• openapi-python-client version 0.17.2

Additional context
my suggestion is, adding branch check value is None before call isoparse function, generate code like this:

  _update_time = d.pop("updateTime", UNSET)
  update_time: Union[Unset, datetime.datetime]
  if isinstance(_update_time, Unset):
      update_time = UNSET
  elif _update_time is None:
      update_time = None
  else:
      update_time = isoparse(_update_time)