message: [hotfix] fix slices where since/until = None
diff: @@ -143,7 +143,7 @@ class BaseViz(object):
  # potential conflicts with column that would be named `from` or `to`
  since = (
  extra_filters.get('__from') or
  - form_data.get("since")
  + form_data.get("since") or ''
  )
  # Backward compatibility hack
  