fix(dataclass): unify and clarify overloads for cls None

[Alviner]

fixes #13189

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pydantic (https://github.com/pydantic/pydantic)
+ pydantic/v1/dataclasses.py:214: error: Incompatible types in assignment (expression has type "type[Any]", variable has type "DataclassProxy")  [assignment]
- pydantic/v1/dataclasses.py:214: error: No overload variant of "dataclass" matches argument types "type[Any]", "bool", "bool", "bool", "bool", "bool", "bool", "bool"  [call-overload]
- pydantic/v1/dataclasses.py:214: note: Possible overload variants:
- pydantic/v1/dataclasses.py:214: note:     def dataclass(None, /) -> Callable[[type[_T]], type[_T]]
- pydantic/v1/dataclasses.py:214: note:     def [_T] dataclass(type[_T], /) -> type[_T]
- pydantic/v1/dataclasses.py:214: note:     def dataclass(*, init: bool = ..., repr: bool = ..., eq: bool = ..., order: bool = ..., unsafe_hash: bool = ..., frozen: bool = ..., match_args: bool = ..., kw_only: bool = ..., slots: bool = ..., weakref_slot: bool = ...) -> Callable[[type[_T]], type[_T]]
+ pydantic/dataclasses.py:256: error: Unused "type: ignore" comment  [unused-ignore]

[Alviner]

@AlexWaygood Hi! Could you please take a look at this PR when you have a moment? If you’re not available, could you tag someone who might be able to review it?
I’d really appreciate the help. Thank you in advance!

[JelleZijlstra]

This now duplicates the overload under if sys.version_info >= (3, 11) branch below.

[JelleZijlstra]

Not sure what is_dataclass adds to this test case.