message: fix since or until is empty value
diff: @@ -173,8 +173,10 @@ class BaseViz(object):
  self.time_shift = utils.parse_human_timedelta(time_shift)
  - from_dttm = utils.parse_human_datetime(since) - self.time_shift
  - to_dttm = utils.parse_human_datetime(until) - self.time_shift
  + since = utils.parse_human_datetime(since)
  + until = utils.parse_human_datetime(until)
  + from_dttm = None if since is None else (since - self.time_shift)
  + to_dttm = None if until is None else (until - self.time_shift)
  if from_dttm and to_dttm and from_dttm > to_dttm:
  raise Exception(_('From date cannot be larger than to date'))
  