@Override
  public Optional<BigInteger> getChainId() {
    return chainId;
  }
Return the transaction chain id (if it exists)

<p>The {@code OptionalInt} will be {@code OptionalInt.empty()} if the transaction is not tied
to a specific chain.

@return the transaction chain id if it exists; otherwise {@code OptionalInt.empty()}
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/core/Transaction.java