Fix query balance of contract token

[horan-geeker]

when I try use

client.EthCall(ethrpc.T{
            To:   contractAddress,
            Data: data,
        }, "latest")

to query balance of erc20 token

type T struct {
	From     string
	To       string
	Gas      int
	GasPrice *big.Int
	Value    *big.Int
	Data     string
	Nonce    int
}

func (t T) MarshalJSON() ([]byte, error) {
	params := map[string]interface{}{
		"from": t.From,
	}
	if t.To != "" {
		params["to"] = t.To
	}
	if t.Gas > 0 {
		params["gas"] = IntToHex(t.Gas)
	}
	if t.GasPrice != nil {
		params["gasPrice"] = BigToHex(*t.GasPrice)
	}
	if t.Value != nil {
		params["value"] = BigToHex(*t.Value)
	}
	if t.Data != "" {
		params["data"] = t.Data
	}
	if t.Nonce > 0 {
		params["nonce"] = IntToHex(t.Nonce)
	}

	return json.Marshal(params)
}

this let t.From = "" and get this json string {"id":1,"jsonrpc":"2.0","method":"eth_call","params":[{"data":"0x70a08231000000000000000000000000fe2e424e2ef9b929157dd525c414f4b128ee1bfb","from":"","to":"0xf4c90e18727c5c76499ea6369c856a6d61d3e92e"},"latest"]} after marshal

finnal error appear

invalid argument 0: hex string has length 0, want 40 for common.Address