Filter by `None`/`Any` for labels/milestones/assignees/weight board config API

See &454 for filtering on different fields in different scenarios.

• Label: None: Return results where there are no labels.
• Label: Any: Return results where that is at least one label.
• Same thing for milestones, assignees, and weight.
• Applies to board config API, i.e. https://docs.gitlab.com/ee/api/boards.html#update-a-board