message: [bugfix] df is None breaks df.empty check
diff: @@ -203,7 +203,7 @@ class BaseViz(object):
  # be considered as the default ISO date format
  # If the datetime format is unix, the parse will use the corresponding
  # parsing logic.
  - if not df.empty:
  + if df is not None and not df.empty:
  if DTTM_ALIAS in df.columns:
  if timestamp_format in ('epoch_s', 'epoch_ms'):
  # Column has already been formatted as a timestamp.
  