Expect an empty string when _submitted_by is None
Expect an empty string when _submitted_by is None

Tests previously expected _submitted_by to export as None when the source
value was None. Change this to an empty string to match the table view,
single-submission view modal, and a forthcoming update to formpack

kobotoolbox/kpi,kobotoolbox/kpi,kobotoolbox/kpi,kobotoolbox/kpi,kobotoolbox/kpi