Allow None in optional dataclass fields

[x0Lazarus]

A dataclass field declared as nickname: Optional[str] = None currently fails validation when its value is None, including when the default is used. Both DataclassSchema and create_dataclass_schema raise MultipleInvalid: expected str because annotation handling drops the None alternative.

Keep that alternative in the field validator so an omitted default or an explicit None can produce the expected dataclass instance. Fields without defaults still require a key, nonnullable fields still reject None, and additional constraints still apply.

Six new cases fail before the change; all eight regression cases pass afterward. On Windows with Python 3.12, the normal suite passes 190 tests, and the existing 19 dataclass tests also pass when invoked explicitly. Black, isort, Flake8 and Mypy pass. The regressions are in the normally collected test file; other platforms and Python versions were not run locally.