Incorrect parsing of null as nullable enum value serialized as strings

Consider the following schema definition (which passes validation with the openapitools/openapi-generator-cli container image):

// [...]
          "deviceEncryptionKeyScheme": {
            "$ref": "#/components/schemas/DeviceEncryptionKeyScheme",
            "nullable": true  // <-- nullability declared
          },

// [...]
      "DeviceEncryptionKeyScheme": {
        "enum": ["Default8"],
        "type": "string"
      },

It looks as if openapi-python-client version 0.23.1 supports deserialization with the deviceEncryptionKeyScheme property being unset, but not it being null:

        _device_encryption_key_scheme = d.pop("deviceEncryptionKeyScheme", UNSET)
        device_encryption_key_scheme: Union[Unset, DeviceEncryptionKeyScheme]
        if isinstance(_device_encryption_key_scheme, Unset):
            device_encryption_key_scheme = UNSET
        else:
            device_encryption_key_scheme = DeviceEncryptionKeyScheme(_device_encryption_key_scheme)

This looks like a bug to me.

使用 openapi-python-client 0.23.1 的行为和所示的生成反序列化代码，复现 issue 中的 nullable enum 情况。从 d.pop 之后对 deviceEncryptionKeyScheme 的处理开始；比较 UNSET 路径和 null 路径。当显式为 null 的值能够成功反序列化，同时未设置的属性仍表示为 UNSET 时，即表示完成。