vm: serialize zero Integer to an empty ByteArray

pkg/vm/emit/bigint.go

@@ -80,10 +80,13 @@ func getEffectiveSize(buf []byte, isNeg bool) int {
 }
 
 // IntToBytes converts integer to a slice in little-endian format.
+// Note: NEO3 serialization differs from default C# BigInteger.ToByteArray()
+//   when n == 0. For zero is equal to empty slice in NEO3.
+// https://github.com/neo-project/neo-vm/blob/master/src/neo-vm/Types/Integer.cs#L16
 func IntToBytes(n *big.Int) []byte {
 	sign := n.Sign()
 	if sign == 0 {
-		return []byte{0}
+		return []byte{}
 	}
 
 	var ws []big.Word

pkg/vm/emit/bigint_test.go

@@ -16,7 +16,7 @@ var testCases = []struct {
 	number int64
 	buf    []byte
 }{
-	{0, []byte{0}},
+	{0, []byte{}},
 	{1, []byte{1}},
 	{-1, []byte{255}},
 	{2, []byte{2}},
@@ -52,7 +52,7 @@ var testCases = []struct {
 	{-9187484529235886209, []byte{127, 127, 127, 127, 127, 127, 127, 128}},
 
 	// https://github.com/dotnet/runtime/blob/master/src/libraries/System.Runtime.Numerics/tests/BigInteger/ToByteArray.cs#L14
-	{0, []byte{0x00}},
+	// {0, []byte{0x00}}, commented because this test check default `BigInteger` serialization, not NEO3 serialization.
 	{3, []byte{0x03}},
 	{128, []byte{0x80, 0x00}},
 	{200, []byte{0xc8, 0x00}},

pkg/vm/vm_test.go

@@ -2213,7 +2213,7 @@ func TestCATInt0ByteArray(t *testing.T) {
 	vm.estack.PushVal([]byte{})
 	runVM(t, vm)
 	assert.Equal(t, 1, vm.estack.Len())
-	assert.Equal(t, &ByteArrayItem{[]byte{0}}, vm.estack.Pop().value)
+	assert.Equal(t, &ByteArrayItem{[]byte{}}, vm.estack.Pop().value)
 }
 
 func TestCATByteArrayInt1(t *testing.T) {