Incorrect parsing of null as nullable enum value serialized as strings

Reproduce el caso de enum nullable del issue usando el comportamiento de openapi-python-client 0.23.1 y el código de deserialización generado que se muestra. Empieza por el manejo de deviceEncryptionKeyScheme después de d.pop; compara la ruta UNSET con la ruta null. El trabajo estará terminado cuando un valor explícitamente null se deserialice correctamente, mientras que una propiedad no establecida siga representándose como UNSET.

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.