Fix backfill when cannot find an event.
Fix backfill when cannot find an event.

`get_pdu` can succeed but return None.

matrix-org/synapse,TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse