fix: tx amount can be nil or zero (#117)

* amount can be nil or zero

* fix cost function

* fix tests

x/evm/types/msg.go

@@ -184,7 +184,9 @@ func (msg *MsgEthereumTx) ChainID() *big.Int {
 // Cost returns amount + gasprice * gaslimit.
 func (msg MsgEthereumTx) Cost() *big.Int {
 	total := msg.Fee()
+	if msg.Data.amount() != nil {
 		total.Add(total, msg.Data.amount())
+	}
 	return total
 }
 

x/evm/types/msg_test.go

@@ -75,7 +75,7 @@ func (suite *MsgsTestSuite) TestMsgEthereumTx_ValidateBasic() {
 		{msg: "pass with recipient - AccessList Tx", to: &suite.to, amount: big.NewInt(100), gasPrice: big.NewInt(0), accessList: &ethtypes.AccessList{}, chainID: big.NewInt(1), expectPass: true},
 		{msg: "pass contract - Legacy Tx", to: nil, amount: big.NewInt(100), gasPrice: big.NewInt(100000), expectPass: true},
 		{msg: "invalid recipient", to: &ethcmn.Address{}, amount: big.NewInt(-1), gasPrice: big.NewInt(1000), expectPass: false},
-		{msg: "nil amount", to: &suite.to, amount: nil, gasPrice: big.NewInt(1000), expectPass: false},
+		{msg: "nil amount", to: &suite.to, amount: nil, gasPrice: big.NewInt(1000), expectPass: true},
 		{msg: "negative amount", to: &suite.to, amount: big.NewInt(-1), gasPrice: big.NewInt(1000), expectPass: true},
 		{msg: "nil gas price", to: &suite.to, amount: big.NewInt(100), gasPrice: nil, expectPass: false},
 		{msg: "negative gas price", to: &suite.to, amount: big.NewInt(100), gasPrice: big.NewInt(-1), expectPass: true},

x/evm/types/tx_data.go

@@ -180,12 +180,8 @@ func (data TxData) Validate() error {
 	}
 
 	amount := data.amount()
-	if amount == nil {
-		return sdkerrors.Wrap(ErrInvalidAmount, "cannot be nil")
-	}
-
 	// Amount can be 0
-	if amount.Sign() == -1 {
+	if amount != nil && amount.Sign() == -1 {
 		return sdkerrors.Wrapf(ErrInvalidAmount, "amount cannot be negative %s", amount)
 	}