Support serialise nested object as nil 

We have to support sending a nested object as `nil` so as to delete the object. 

```
  objectUpdate(input: $input) {
        userErrors { field, message }
        object {
          id
          title
          suboject {
            id
            name
          }
        }
      }
    }
```

To do that we have to modify the `serialise` function. 
