Handle Nethermind backwards sync

[ajsutton]

Description

We noticed sth like this on teku <> nethermind

09:18:09.096 FATAL - Exiting due to fatal error in Eth1DepositManager java.util.concurrent.CompletionException: tech.pegasys.teku.infrastructure.exceptions.FatalServiceFailureException: java.util.concurrent.CompletionException: tech.pegasys.teku.beacon.pow.exception.FailedToFindMinGenesisBlockException: Failed to retrieve min genesis block. Check that your eth1 node is fully synced. (See log file for full stack trace) Teku is shutting down Thread pool did not terminate and the problem is that we're still downloading old blocks and receipts Nethermind is syncing to head at first and later we're downloading history of the chain. So we could response with null on old blocks