fix: understandable error when there is no fee provided but required

x/tax/keeper/custom_tx_fee_checker.go

@@ -43,6 +43,11 @@ func (k Keeper) CustomTxFeeChecker(ctx sdk.Context, tx sdk.Tx) (sdk.Coins, int64
 			baseDenom := k.GetParams(ctx).BaseDenom
 			minimumFeeRequired := sdk.NewCoin(baseDenom, minGasPrices[0].Amount.Mul(glDec).Ceil().RoundInt())
 
+			// if there are no fees provided
+			if feeCoins.Len() == 0 {
+				return nil, 0, errors.Wrapf(sdkerrors.ErrInsufficientFee, "insufficient fees; got: %s required: %s", feeCoins, requiredFees)
+			}
+
 			// if there are no fees paid in the base asset
 			if ok, _ := feeCoins.Find(baseDenom); !ok {
 				// Get Fee Param for select dex based on the feeCoins provided

x/tax/keeper/custom_tx_fee_checker_test.go

@@ -136,7 +136,7 @@ func TestCustomTxFeeCheckerSuccessfulInNLS(t *testing.T) {
 }
 
 // Fail to pay fees in unsupported denom.
-func TestCustomTxFeeCheckerSuccessfulInUnsupportedDenom(t *testing.T) {
+func TestCustomTxFeeCheckerFailWhenUnsupportedDenom(t *testing.T) {
 	taxKeeper, ctx, _ := keepertest.TaxKeeper(t, true, sdk.DecCoins{sdk.NewDecCoin("unls", sdk.NewInt(1))})
 	// create a new CustomTxFeeChecker
 	feeTx := keepertest.MockFeeTx{
@@ -240,3 +240,13 @@ func TestCustomTxFeeCheckerFailOnZeroFees(t *testing.T) {
 	_, _, err := taxKeeper.CustomTxFeeChecker(ctx, feeTx)
 	require.Error(t, err)
 }
+
+// Successfully pay fees in unls which represents NLS. Minimum gas prices set to unls.
+func TestCustomTxFeeCheckerFailWhenEmptyFee(t *testing.T) {
+	taxKeeper, ctx, _ := keepertest.TaxKeeper(t, true, sdk.DecCoins{sdk.NewDecCoin("unls", sdk.NewInt(1))})
+	// create a new CustomTxFeeChecker
+	feeTx := keepertest.MockFeeTx{}
+
+	_, _, err := taxKeeper.CustomTxFeeChecker(ctx, feeTx)
+	require.Error(t, err)
+}