Fix `/messages` throwing a 500 when querying for non-existent room (#12683)

Fix https://github.com/matrix-org/synapse/issues/12678

Complement test added:  https://github.com/matrix-org/complement/pull/369

**Before:** 500 internal server error

**After:** According to the [spec](https://spec.matrix.org/latest/client-server-api/#get_matrixclientv3roomsroomidmessages), calling `/messages` against a non-existent `room_id` should throw a 403 forbidden (since you're not part of the room). This also matches the behavior before https://github.com/matrix-org/synapse/pull/12370 which regressed Synapse to the 500 behavior.
```json
{
    "errcode": "M_FORBIDDEN",
    "error": "User @test:my.synapse.server not in room !dne:my.synapse.server, and room previews are disabled"
}
```

1 files changed, 1 insertions, 1 deletions

diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index 7ee3340373..2e30180094 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -448,7 +448,7 @@ class PaginationHandler:
             )
             # We expect `/messages` to use historic pagination tokens by default but
             # `/messages` should still works with live tokens when manually provided.
-            assert from_token.room_key.topological
+            assert from_token.room_key.topological is not None
 
         if pagin_config.limit is None:
             # This shouldn't happen as we've set a default limit before this