beacon: fix checks for withdrawalsRoot and withdrawals list

beacon/engine/types.go

@@ -275,15 +275,15 @@ func ExecutableDataToBlockNoHash(data ExecutableData, versionedHashes []common.H
 	// ExecutableData before withdrawals are enabled by marshaling
 	// Withdrawals as the json null value.
 	var withdrawalsRoot *common.Hash
-	if config.IsOptimismIsthmus(data.Timestamp) && data.WithdrawalsRoot != nil {
-		if data.Withdrawals == nil || len(data.Withdrawals) != 0 {
-			return nil, fmt.Errorf("attribute WithdrawalsRoot was set. Expecting non-nil empty withdrawals list, but got %v", data.Withdrawals)
-		}
-		h := *data.WithdrawalsRoot // copy, avoid any sharing of memory
-		withdrawalsRoot = &h
-	} else if data.Withdrawals != nil {
+	if config.IsOptimismIsthmus(data.Timestamp) && data.WithdrawalsRoot == nil {
+		return nil, fmt.Errorf("attribute WithdrawalsRoot is required for Isthmus blocks")
+	}
+	if data.Withdrawals != nil {
 		h := types.DeriveSha(types.Withdrawals(data.Withdrawals), trie.NewStackTrie(nil))
 		withdrawalsRoot = &h
+	} else if data.WithdrawalsRoot != nil {
+		h := *data.WithdrawalsRoot // copy, avoid any sharing of memory
+		withdrawalsRoot = &h
 	}
 	// Compute requestsHash if any requests are non-nil.
 	var (