accounts/external: handle 0 chainid as not-set for the Clef API

accounts/external/backend.go

@@ -218,17 +218,17 @@ func (api *ExternalSigner) SignTx(account accounts.Account, tx *types.Transactio
 		args.MaxFeePerGas = (*hexutil.Big)(tx.GasFeeCap())
 		args.MaxPriorityFeePerGas = (*hexutil.Big)(tx.GasTipCap())
 	default:
-		return nil, fmt.Errorf("Unsupported tx type %d", tx.Type())
+		return nil, fmt.Errorf("unsupported tx type %d", tx.Type())
 	}
 	// We should request the default chain id that we're operating with
 	// (the chain we're executing on)
-	if chainID != nil {
+	if chainID != nil && chainID.Sign() != 0 {
 		args.ChainID = (*hexutil.Big)(chainID)
 	}
 	if tx.Type() != types.LegacyTxType {
 		// However, if the user asked for a particular chain id, then we should
 		// use that instead.
-		if tx.ChainId() != nil {
+		if tx.ChainId().Sign() != 0 {
 			args.ChainID = (*hexutil.Big)(tx.ChainId())
 		}
 		accessList := tx.AccessList()