return check for nil tx in rpc block verification

op-service/sources/types.go

@@ -196,6 +196,11 @@ func (block *rpcBlock) verify() error {
 	if computed := block.computeBlockHash(); computed != block.Hash {
 		return fmt.Errorf("failed to verify block hash: computed %s but RPC said %s", computed, block.Hash)
 	}
+	for i, tx := range block.Transactions {
+		if tx == nil {
+			return fmt.Errorf("block tx %d is nil", i)
+		}
+	}
 	if computed := types.DeriveSha(types.Transactions(block.Transactions), trie.NewStackTrie(nil)); block.TxHash != computed {
 		return fmt.Errorf("failed to verify transactions list: computed %s but RPC said %s", computed, block.TxHash)
 	}