fix(rpc/types): catch invalid values in CheckTxFee

This change catches invalid values in CheckTxFee
and returns an error for:
* nil gasPrice
* negative gasPrice

which improves the reliability of the system.

Fixes evmos#2413
Fixes evmos#2414

rpc/types/utils_test.go

@@ -0,0 +1,17 @@
+package types
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestCheckTxFeeInvalidArgs(t *testing.T) {
+	if err := CheckTxFee(nil, 10, 100); err == nil {
+		t.Fatal("expecting a non-nil error")
+	}
+
+	gp := big.NewInt(-1)
+	if err := CheckTxFee(gp, 10, 100); err == nil {
+		t.Fatal("expecting a non-nil error")
+	}
+}