Fix messages from multiple senders in historical chunk (MSC2716) (#10276)

Fix messages from multiple senders in historical chunk. This also means that an app service does not need to define `?user_id` when using this endpoint.

Follow-up to https://github.com/matrix-org/synapse/pull/9247

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716