core: avoid nil values during natives manifest marshalling

pkg/core/interop/context.go

@@ -140,6 +140,7 @@ func NewContractMD(name string, id int32) *ContractMD {
 	// Therefore values are taken from C# node.
 	c.NEF.Header.Compiler = "neo-core-v3.0"
 	c.NEF.Header.Magic = nef.Magic
+	c.NEF.Tokens = []nef.MethodToken{} // avoid `nil` result during JSON marshalling
 	c.Hash = state.CreateContractHash(util.Uint160{}, 0, c.Name)
 	c.Manifest = *manifest.DefaultManifest(name)
 

pkg/core/native/native_nep17.go

@@ -294,6 +294,9 @@ func (c *nep17TokenNative) addTokens(ic *interop.Context, h util.Uint160, amount
 }
 
 func newDescriptor(name string, ret smartcontract.ParamType, ps ...manifest.Parameter) *manifest.Method {
+	if len(ps) == 0 {
+		ps = []manifest.Parameter{}
+	}
 	return &manifest.Method{
 		Name:       name,
 		Parameters: ps,

pkg/smartcontract/nef/nef_test.go

@@ -69,6 +69,14 @@ func TestEncodeDecodeBinary(t *testing.T) {
 		expected.Header.Magic = Magic
 		testserdes.EncodeDecodeBinary(t, expected, &File{})
 	})
+	t.Run("positive with empty tokens", func(t *testing.T) {
+		expected.Script = script
+		// Check `Tokens` won't be deserialized to `nil`. We expect them to be non-nil slice in the related code.
+		expected.Tokens = []MethodToken{}
+		expected.Checksum = expected.CalculateChecksum()
+		expected.Header.Magic = Magic
+		testserdes.EncodeDecodeBinary(t, expected, &File{})
+	})
 	t.Run("invalid reserved bytes", func(t *testing.T) {
 		expected.Script = script
 		expected.Tokens = expected.Tokens[:0]