Fix typing bug for older python versions

[winterchristian]

Description

This fixes a typing bug introduced in #3416 that only affects older python versions (< python3.10).

Motivation and Context

In #3416 the type hint str was changed to str | None. This works fine for all newer python versions >= 3.10. For the older version law uses the typing-extensions library to introduce typing support for older python versions <3.10.
This library does not support the short equivalent str | None instead of Union[str, None]. In this case that one type is None it is also equivalent to Optional[str].

So this PR just replaces str | None with Optional[str].

Have you tested this? If so, how?

As this is a trivial change, I tested it only locally with my setup using python3.9 and python3.13.

[dlstadther]

Luigi removed official support for 3.9 with 6959c1f so this PR is N/A.

[winterchristian]

Hi @dlstadther your right I missed that. Should I then close this PR, or should it still be merged out of consistency reasons? The whole file uses Union[] and Optional[] instead of |.

[dlstadther]

There are many places in the repo which haven't been moved away from typing.Union and/or typing.Optional (i actually commented a similar mention on another recent PR). These typing updates (moving to the 3.10+ style) can be made in a separate PR.

I'll close this PR since its intention is n/a. Sorry for the confusion.