feat(ethapi): omits additional fields when empty

Omits `feeCurrency`, `gatewayFeeRecipient`, and `gatewayFee` from the response when they are empty (effectively on Ethereum-compatible transactions).

When I call `eth_gasTransactionByHash` on an EIP-1559 transaction, the response object includes `feeCurrency`, `gatewayFeeRecipient`, and `gatewayFee` although they aren't relevant.

internal/ethapi/api.go

@@ -1288,9 +1288,9 @@ type RPCTransaction struct {
 	GasPrice            *hexutil.Big      `json:"gasPrice"`
 	GasFeeCap           *hexutil.Big      `json:"maxFeePerGas,omitempty"`
 	GasTipCap           *hexutil.Big      `json:"maxPriorityFeePerGas,omitempty"`
-	FeeCurrency         *common.Address   `json:"feeCurrency"`
-	GatewayFeeRecipient *common.Address   `json:"gatewayFeeRecipient"`
-	GatewayFee          *hexutil.Big      `json:"gatewayFee"`
+	FeeCurrency         *common.Address   `json:"feeCurrency,omitempty"`
+	GatewayFeeRecipient *common.Address   `json:"gatewayFeeRecipient,omitempty"`
+	GatewayFee          *hexutil.Big      `json:"gatewayFee,omitempty"`
 	Hash                common.Hash       `json:"hash"`
 	Input               hexutil.Bytes     `json:"input"`
 	Nonce               hexutil.Uint64    `json:"nonce"`