Fix a NPE when the expected array is actually null

Don't crash with a NPE when the expected array is actually null, just return null.

```json
{
  "success": true,
  "data": {
    "user": 3,
    "own": null,
    "passes": null,
    "completed": null
  },
  "version": 1371160528774
}
```

Without this fix, the path `$.data.passes[0].id` gets a NPE.