Disallow outliers from being used with the gap checks

See #14096 (comment)

synapse/storage/databases/main/events_worker.py

@@ -1976,12 +1976,17 @@ async def is_event_next_to_backward_gap(self, event: EventBase) -> bool:
 
         Args:
             room_id: room where the event lives
-            event_id: event to check
+            event_id: event to check (can't be an `outlier`)
 
         Returns:
             Boolean indicating whether it's an extremity
         """
 
+        assert not event.internal_metadata.is_outlier(), (
+            "is_event_next_to_backward_gap(...) can not be used with `outlier` events. "
+            "This function relies on `event_backward_extremities` which won't be filled in for `outliers."
+        )
+
         def is_event_next_to_backward_gap_txn(txn: LoggingTransaction) -> bool:
             # If the event in question has any of its prev_events listed as a
             # backward extremity, it's next to a gap.
@@ -2031,12 +2036,17 @@ async def is_event_next_to_forward_gap(self, event: EventBase) -> bool:
 
         Args:
             room_id: room where the event lives
-            event_id: event to check
+            event_id: event to check (can't be an `outlier`)
 
         Returns:
             Boolean indicating whether it's an extremity
         """
 
+        assert not event.internal_metadata.is_outlier(), (
+            "is_event_next_to_forward_gap(...) can not be used with `outlier` events. "
+            "This function relies on `event_edges` and `event_forward_extremities` which won't be filled in for `outliers."
+        )
+
         def is_event_next_to_gap_txn(txn: LoggingTransaction) -> bool:
             # If the event in question is a forward extremity, we will just
             # consider any potential forward gap as not a gap since it's one of