Fix implicit None in OrderedSet.__init__

[Glinte]

Implicit None is an error in typecheckers now

mypy

ordered_set\__init__.py:66: error: Incompatible default for argument "initial" (default has type "None", argument has type "AbstractSet[T] | Sequence[T] | Iterable[T]")  [assignment]
ordered_set\__init__.py:66: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
ordered_set\__init__.py:66: note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase

pyright

  ordered-set\test\test_ordered_set.py:91:12 - error: Argument of type "None" cannot be assigned to parameter "initial" of type "OrderedSetInitializer[T@OrderedSet]" in function "__init__"
    Type "None" is not assignable to type "OrderedSetInitializer[T@OrderedSet]"
      "None" is not assignable to "AbstractSet[T@OrderedSet]"
      "None" is incompatible with protocol "Iterable[T@OrderedSet]"
        "__iter__" is not present
      "None" is not assignable to "Sequence[T@OrderedSet]" (reportArgumentType)