client.projects.exports.download returns None

I have the following code:

```python
from label_studio_sdk import ExportCreate, ExportConvert
from label_studio_sdk.client import LabelStudio

LABEL_STUDIO_URL = '...'
API_KEY = '...'
PROJECT_ID = 1

ls = LabelStudio(base_url=LABEL_STUDIO_URL, api_key=API_KEY)

export_task = ls.projects.exports.create(
id=PROJECT_ID,
request=ExportCreate()
)

convert_task = ls.projects.exports.convert(
id=PROJECT_ID,
export_pk=export_task.id,
request=ExportConvert(
export_type="YOLO"
)
)

data = ls.projects.exports.download(id=PROJECT_ID, export_pk=export_task.id, export_type="YOLO")
```

And the value of data is `None`, I am testing it on notebook so I am sure the task is in `completed` state.