StationOverviewFields: domain field not nullable

When executing this code:

```python
import json
from pprint import pprint
from deutschland.ladestationen.api import default_api

api_instance = default_api.DefaultApi()
geometry = {
"xmin": 648310.999314399,
"xmax": 1708898.35458854,
"ymin": 5998767.0224341,
"ymax": 7353682.52593813,
}
api_response = api_instance.query_get(json.dumps(geometry), f="json")
pprint(api_response)
```

I get this error:

```python
deutschland.ladestationen.exceptions.ApiTypeError: Invalid type for variable 'domain'. Required value type is str and passed type was NoneType at ['received_data']['fields'][0]['domain']
```

So it seems that the `domain` field isn't actually nullable as described in the OpenAPI spec.