Fix ListFilter's expected_parameters return type

[pheki]

Currently ListFilter's expected_parameters' return type is defined as list[str] | None, which is incorrect in my opinion as:

• If a FieldListFilter subclass returns None it will raise here as None is not iterable
• The default implementation of SimpleListFilter returns [None], which is not allowed by the current return type.

This PR changes the return type to list[str | None].

Related issues

• None, but I can open one if required

[intgr]

Thanks!