fix optional type hints

[juanitorduz]

hey! this PR just correct some type hints: From the documentation:

You can use the Optional type modifier to define a type variant that allows None, such as Optional[int] (Optional[X] is the preferred shorthand for Union[X, None])

This means that a parameter with a default value is not an Optional type. We will capture these type of errors once we set up mypy.
In addition, we should not use mutable structures as defaults. It is better to set None as default, see https://florimond.dev/en/posts/2018/08/python-mutable-defaults-are-the-source-of-all-evil/