l2geth: docall protect nil block (#736)

* l2geth: protect nil in eth_call

* chore: add changeset

.changeset/forty-hotels-brake.md

+---
+'@eth-optimism/l2geth': patch
+---
+
+Protect a possible `nil` reference in `eth_call` when the blockchain is empty

l2geth/internal/ethapi/api.go

@@ -937,6 +937,7 @@ func DoCall(ctx context.Context, b Backend, args CallArgs, blockNrOrHash rpc.Blo
 		if err != nil {
 			return nil, 0, false, err
 		}
+		if block != nil {
 			txs := block.Transactions()
 			if header.Number.Uint64() != 0 {
 				if len(txs) != 1 {
@@ -946,6 +947,7 @@ func DoCall(ctx context.Context, b Backend, args CallArgs, blockNrOrHash rpc.Blo
 				blockNumber = tx.L1BlockNumber()
 				timestamp = new(big.Int).SetUint64(tx.L1Timestamp())
 			}
+		}
 		msg, err = core.EncodeSimulatedMessage(msg, timestamp, blockNumber, executionManager, stateManager)
 		if err != nil {
 			return nil, 0, false, err