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](e0fdb862cb/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](e0fdb862cb/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.
Mathieu Velten