Fix exception when using MSC3030 to look for remote federated events before room creation

[MadLittleMods]

Fix exception when using MSC3030 to look for remote federated events before room creation.

Complement tests: matrix-org/complement#405

This happens when you have some messages imported before the room is created.
Then use MSC3030 to look backwards before the room creation from a remote
federated server. The server won't find anything locally, but will ask over
federation which will have the remote event. The previous logic would
choke on not having the local event assigned.

Failed to fetch /timestamp_to_event from hs2 because of exception(UnboundLocalError) local variable 'local_event' referenced before assignment args=("local variable 'local_event' referenced before assignment",)

Dev notes

synapse_main | 2022-07-06 13:32:48,807 - synapse.handlers.room - 1369 - INFO - GET-6 - get_event_for_timestamp: locally, we found event_id=None closest to timestamp=1657114367164
synapse_main | 2022-07-06 13:32:48,808 - synapse.handlers.room - 1404 - INFO - GET-6 - get_event_for_timestamp: locally, we found event_id=None closest to timestamp=1657114367164 which is next to a gap in event history so we're asking other homeservers first

synapse_main | 2022-07-06 13:32:48,900 - synapse.handlers.room - 1425 - INFO - GET-6 - get_event_for_timestamp: response from domain(hs1)=TimestampToEventResponse(event_id='$6N0qwxOfLXTf7BZRzc5J9LXEPz1dXbWhD1IeAhecNRM', origin_server_ts=1641189600000, data={'event_id': '$6N0qwxOfLXTf7BZRzc5J9LXEPz1dXbWhD1IeAhecNRM', 'origin_server_ts': 1641189600000})
synapse_main | 2022-07-06 13:32:48,903 - synapse.handlers.room - 1464 - WARNING - GET-6 - Failed to fetch /timestamp_to_event from hs1 because of exception(UnboundLocalError) local variable 'local_event' referenced before assignment args=("local variable 'local_event' referenced before assignment",)

The got is because it has those state events locally even though that user joined after those events. We want the imported event which has a timestamp before the creation event.

                   -- oldest events --
                   $azd78vChQWi-mL8ujkvYDaGUoBgf5LeiJtcd96SjuVw (1657114367199) - m.room.create
                   $nrXeQWObbNdV1bdT_aV5imUHm-Y7RJjv4t2Bf2Q5t04 (1657114367381) - m.room.member
                   $_8LZSYqibJvowol8p9bkd-tFzUEKrUZ40I6ZdBXLu88 (1657114367431) - m.room.power_levels
                   $ZPCpSJy-24c_WIHTr3RFmVETRE5yzHl9xT27ek5DkZ4 (1657114367479) - m.room.join_rules
             (got) $_zb4erdxtTYrqOvRdqxTKONKzEk8iq9iPDfjx9Ey6VY (1657114367522) - m.room.history_visibility
                   $l8DQs4X8PiZqqsHCDHMnkjmPMJruwtE3biWXW7TJPCc (1657114367572) - m.room.message
                   $gQTxarzOc3rzF1FLr3pcCNyxJtMdwbfnSZraLLXGa8U (1657114367649) - m.room.message
                   $kRFvZBZT9p4nLqSGRfFmTUER-GKosW6aYpL_DdSKbBM (1657114367702) - m.room.member
  (want, imported) $6N0qwxOfLXTf7BZRzc5J9LXEPz1dXbWhD1IeAhecNRM (1641189600000) - m.room.message
                   $8h4X8YiVmOqTj_HSRUCY4LuiO8oA3xOVR0WjGMLPgJU (1657114368559) - m.room.member
                   -- newest events --

Pull Request Checklist

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[MadLittleMods]

Thanks for the review @squahtx 🐰