smartcontract: add JSON marshal/unmarshal for InteropType

We actually have to do that in order to answer getapplicationlog requests for
transactions that leave some interop items on the stack. It follows the same
logic our binary serializer/deserializes does leaving the type and stripping
the value (whatever that is).

pkg/smartcontract/parameter_test.go

@@ -122,6 +122,13 @@ var marshalJSONTestCases = []struct {
 		},
 		result: `{"type":"Hash256","value":"0xf037308fa0ab18155bccfc08485468c112409ea5064595699e98c545f245f32d"}`,
 	},
+	{
+		input: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+		result: `{"type":"InteropInterface","value":null}`,
+	},
 }
 
 var marshalJSONErrorCases = []Parameter{
@@ -129,10 +136,6 @@ var marshalJSONErrorCases = []Parameter{
 		Type:  UnknownType,
 		Value: nil,
 	},
-	{
-		Type:  InteropInterfaceType,
-		Value: nil,
-	},
 	{
 		Type:  IntegerType,
 		Value: math.Inf(1),
@@ -252,6 +255,27 @@ var unmarshalJSONTestCases = []struct {
 		},
 		input: `{"type":"PublicKey","value":"03b3bf1502fbdc05449b506aaf04579724024b06542e49262bfaa3f70e200040a9"}`,
 	},
+	{
+		input: `{"type":"InteropInterface","value":null}`,
+		result: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+	},
+	{
+		input: `{"type":"InteropInterface","value":""}`,
+		result: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+	},
+	{
+		input: `{"type":"InteropInterface","value":"Hundertwasser"}`,
+		result: Parameter{
+			Type:  InteropInterfaceType,
+			Value: nil,
+		},
+	},
 }
 
 var unmarshalJSONErrorCases = []string{
@@ -272,8 +296,6 @@ var unmarshalJSONErrorCases = []string{
 	`{"type": "Map","value": ["key": {}]}`, // incorrect Map value
 	`{"type": "Map","value": ["key": {"type":"String", "value":"qwer"}, "value": {"type":"Boolean"}]}`, // incorrect Map Value value
 	`{"type": "Map","value": ["key": {"type":"String"}, "value": {"type":"Boolean", "value":true}]}`,   // incorrect Map Key value
-
-	`{"type": "InteropInterface","value": ""}`, // ununmarshable type
 }
 
 func TestParam_UnmarshalJSON(t *testing.T) {