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

@@ -937,14 +937,16 @@ func DoCall(ctx context.Context, b Backend, args CallArgs, blockNrOrHash rpc.Blo
 		if err != nil {
 			return nil, 0, false, err
 		}
-		txs := block.Transactions()
-		if header.Number.Uint64() != 0 {
-			if len(txs) != 1 {
-				return nil, 0, false, fmt.Errorf("block %d has more than 1 transaction", header.Number.Uint64())
+		if block != nil {
+			txs := block.Transactions()
+			if header.Number.Uint64() != 0 {
+				if len(txs) != 1 {
+					return nil, 0, false, fmt.Errorf("block %d has more than 1 transaction", header.Number.Uint64())
+				}
+				tx := txs[0]
+				blockNumber = tx.L1BlockNumber()
+				timestamp = new(big.Int).SetUint64(tx.L1Timestamp())
 			}
-			tx := txs[0]
-			blockNumber = tx.L1BlockNumber()
-			timestamp = new(big.Int).SetUint64(tx.L1Timestamp())
 		}
 		msg, err = core.EncodeSimulatedMessage(msg, timestamp, blockNumber, executionManager, stateManager)
 		if err != nil {