op-node: sanity check l1Blocks length, fix import

op-node/rollup/derive/batch_queue.go

@@ -79,6 +79,9 @@ func (bq *BatchQueue) popNextBatch(safeL2Head eth.L2BlockRef) *SingularBatch {
 }
 
 func (bq *BatchQueue) advanceEpochMaybe(nextBatch *SingularBatch) {
+	if len(bq.l1Blocks) == 0 {
+		return
+	}
 	if nextBatch.GetEpochNum() == rollup.Epoch(bq.l1Blocks[0].Number)+1 {
 		// Advance epoch if necessary
 		bq.l1Blocks = bq.l1Blocks[1:]

op-node/rollup/derive/batch_queue_test.go

@@ -9,10 +9,9 @@ import (
 	"math/rand"
 	"testing"
 
-	"github.com/ethereum/go-ethereum/core/types"
-
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/stretchr/testify/require"