Support nullable `fromJson`

I have a type I get from a php project, which looks a bit like this:
```json
{
"object": "user",
"attributes": {...}
}
```

And sometimes, that object can be nested

```dart
{
"object": "user",
"attributes": {
"thing": {
"object": "item"
"attributes": {...}
}
}
}
```
This is fine.

What _isnt_ fine, however, is the `null_resource` object, which looks like this:
```dart
{
"object": "null_resource",
"attributes": null // <<<<<<<<<<
}
```
uh oh. I dont want two layers of null!