Provide meaningful response message for void methods

Now the invocation returns

```
{
  "result" : "null"
}
```

There are three ways how to resolve the issue
- return nothing
- make result optional and return empty JSON object
- return more complex return message like 

```
{
  "status" : "ok",
  "result" : "....",
}
```

or

```
{
  "status" : "ok",
}
```
