rpc: always marshal FaultException

pkg/rpc/response/result/invoke.go

@@ -66,7 +66,7 @@ type invokeAux struct {
 	GasConsumed    int64                     `json:"gasconsumed,string"`
 	Script         []byte                    `json:"script"`
 	Stack          json.RawMessage           `json:"stack"`
-	FaultException string                    `json:"exception,omitempty"`
+	FaultException *string                   `json:"exception"`
 	Notifications  []state.NotificationEvent `json:"notifications"`
 	Transaction    []byte                    `json:"tx,omitempty"`
 	Diagnostics    *InvokeDiag               `json:"diagnostics,omitempty"`
@@ -145,16 +145,19 @@ arrloop:
 	if r.Transaction != nil {
 		txbytes = r.Transaction.Bytes()
 	}
-	return json.Marshal(&invokeAux{
+	aux := &invokeAux{
 		GasConsumed:   r.GasConsumed,
 		Script:        r.Script,
 		State:         r.State,
 		Stack:         st,
-		FaultException: r.FaultException,
 		Notifications: r.Notifications,
 		Transaction:   txbytes,
 		Diagnostics:   r.Diagnostics,
-	})
+	}
+	if len(r.FaultException) != 0 {
+		aux.FaultException = &r.FaultException
+	}
+	return json.Marshal(aux)
 }
 
 // UnmarshalJSON implements the json.Unmarshaler.
@@ -207,7 +210,9 @@ func (r *Invoke) UnmarshalJSON(data []byte) error {
 	r.GasConsumed = aux.GasConsumed
 	r.Script = aux.Script
 	r.State = aux.State
-	r.FaultException = aux.FaultException
+	if aux.FaultException != nil {
+		r.FaultException = *aux.FaultException
+	}
 	r.Notifications = aux.Notifications
 	r.Transaction = tx
 	r.Diagnostics = aux.Diagnostics

pkg/rpc/response/result/invoke_test.go

@@ -40,6 +40,7 @@ func TestInvoke_MarshalJSON(t *testing.T) {
 			{"type":"Integer","value":"1"}
 		],
 		"notifications":[],
+		"exception": null,
 		"tx":"` + base64.StdEncoding.EncodeToString(tx.Bytes()) + `"
 }`
 	require.JSONEq(t, expected, string(data))