Fix bug when rejecting withdrew invite with a third_party_rules module (#17930)

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](https://github.com/element-hq/synapse/blob/e0fdb862cbbddc920a30233024eb99038ee2fb28/synapse/module_api/callbacks/third_party_event_rules_callbacks.py#L285)
on the context to calculate the state to pass at `check_event_allowed`
callbacks.

The context for outliers is defined
[here](https://github.com/element-hq/synapse/blob/e0fdb862cbbddc920a30233024eb99038ee2fb28/synapse/events/snapshot.py#L168),
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.