Superset: ingestion of the empty dashboard fails

[grumbler]

When dashboard is empty and doesn't contain any charts ingestion fails here:

        raw_position_data = dashboard_data.get("position_json", "{}")
        position_data = json.loads(raw_position_data)

position_json is None in this case

Expected behavior

Import empty dashboard without charts

[shirshanka]

Fixed via #3714 thank you!