Fix bug when rejecting withdrew invite with a third_party_rules module

[MatMaul]

When rejecting a withdrew invite through federation, an out of band event needs to be created.

When doing so with a third_party_rules module installed, get_prev_state_ids is called on the context to calculate the state to pass at check_event_allowed callbacks.

The context for outliers is defined here, and state_group_before_event is None.

This change makes the behavior of get_prev_state_ids and get_current_state_ids match the one presented in the docstring regarding null state_group.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file.
• Code style is correct
  (run the linters)

[reivilibre]

Makes sense; just a small change