Fix "implicit optional", e.g. arg: int = None

[andrew222651]

The latest version of Pyright makes calling a function with an "implicit optional" a type error. An "implicit optional" is like the following code, where a parameter is annotated with a type that doesn't include None but the default value is None:

def foo(arg: int = None):
    pass

More info here: https://docs.astral.sh/ruff/rules/implicit-optional/

This PR adds typing.Optional where necessary.