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.

Author: MatMaul

changelog.d/17930.bugfix

@@ -0,0 +1 @@
+Fix bug when rejecting withdrew invite with a third_party_rules module, where the invite would be stuck for the client.

synapse/events/snapshot.py

@@ -248,21 +248,20 @@ def state_group(self) -> Optional[int]:
     @tag_args
     async def get_current_state_ids(
         self, state_filter: Optional["StateFilter"] = None
-    ) -> Optional[StateMap[str]]:
+    ) -> StateMap[str]:
         """
         Gets the room state map, including this event - ie, the state in ``state_group``
 
         It is an error to access this for a rejected event, since rejected state should
         not make it into the room state. This method will raise an exception if
         ``rejected`` is set.
 
+        It is also an error to access this for an outlier event.
+
         Arg:
            state_filter: specifies the type of state event to fetch from DB, example: EventTypes.JoinRules
 
         Returns:
-            Returns None if state_group is None, which happens when the associated
-            event is an outlier.
-
             Maps a (type, state_key) to the event ID of the state event matching
             this tuple.
         """
@@ -300,7 +299,8 @@ async def get_prev_state_ids(
             this tuple.
         """
 
-        assert self.state_group_before_event is not None
+        if self.state_group_before_event is None:
+            return {}
         return await self._storage.state.get_state_ids_for_group(
             self.state_group_before_event, state_filter
         )