Keep null in list

Currently if a json array contains a value equal to null, the value will be omitted.
But at some points, one would like to handle it better:
- Remove it
- Keep it
 If we have the following json:
```
{
  "test":[null, null, null, {"src": "test"}]
}
```

In output we could have:
- "test":[null, null, null, {"src": "test"}]
- "test":["src": "test"}]
