smartcontract: update (Parameter).MarshalJSON method

MarshalJSON should be defined on structure (not pointer), as we use
structures to marshal parameters (e.g. in NotificationEvent and
Invoke of RPC result package) and never use pointers for that purpose.

Also added marshalling of nil array into `[]` instead of `null` to
follow C# implementation.

pkg/core/native/blocked_accounts_test.go

@@ -1,9 +1,11 @@
 package native
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/nspcc-dev/neo-go/pkg/internal/testserdes"
+	"github.com/nspcc-dev/neo-go/pkg/smartcontract"
 	"github.com/nspcc-dev/neo-go/pkg/util"
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 	"github.com/stretchr/testify/require"
@@ -51,3 +53,12 @@ func TestToStackItem(t *testing.T) {
 	actual = stackitem.NewArray([]stackitem.Item{})
 	require.Equal(t, expected.ToStackItem(), actual)
 }
+
+func TestMarshallJSON(t *testing.T) {
+	ba := &BlockedAccounts{}
+	p := smartcontract.ParameterFromStackItem(ba.ToStackItem(), make(map[stackitem.Item]bool))
+	actual, err := json.Marshal(p)
+	require.NoError(t, err)
+	expected := `{"type":"Array","value":[]}`
+	require.Equal(t, expected, string(actual))
+}

pkg/smartcontract/parameter.go

@@ -57,7 +57,7 @@ type rawParameter struct {
 }
 
 // MarshalJSON implements Marshaler interface.
-func (p *Parameter) MarshalJSON() ([]byte, error) {
+func (p Parameter) MarshalJSON() ([]byte, error) {
 	var (
 		resultRawValue json.RawMessage
 		resultErr      error
@@ -83,7 +83,11 @@ func (p *Parameter) MarshalJSON() ([]byte, error) {
 		}
 	case ArrayType:
 		var value = p.Value.([]Parameter)
+		if value == nil {
+			resultRawValue, resultErr = json.Marshal([]Parameter{})
+		} else {
 			resultRawValue, resultErr = json.Marshal(value)
+		}
 	case MapType:
 		ppair := p.Value.([]ParameterPair)
 		resultRawValue, resultErr = json.Marshal(ppair)

pkg/smartcontract/parameter_test.go

@@ -131,6 +131,13 @@ var marshalJSONTestCases = []struct {
 		},
 		result: `{"type":"InteropInterface","value":null}`,
 	},
+	{
+		input: Parameter{
+			Type:  ArrayType,
+			Value: []Parameter{},
+		},
+		result: `{"type":"Array","value":[]}`,
+	},
 }
 
 var marshalJSONErrorCases = []Parameter{
@@ -146,7 +153,7 @@ var marshalJSONErrorCases = []Parameter{
 
 func TestParam_MarshalJSON(t *testing.T) {
 	for _, tc := range marshalJSONTestCases {
-		res, err := json.Marshal(&tc.input)
+		res, err := json.Marshal(tc.input)
 		assert.NoError(t, err)
 		var actual, expected Parameter
 		assert.NoError(t, json.Unmarshal(res, &actual))