message: Handle empty dataframes in TableViz
  TableViz fails to display empty dataframes returning an error like:
  "None of [Index(['project', 'count'], dtype='object')] are in the [columns]"
  The behavior has been observed while testing 0.36.0rc3 with
  Druid datasources.
  issue:
diff: @@ -625,6 +625,7 @@ class TableViz(BaseViz):
  self.form_data.get("percent_metrics") or []
  )
  + if not df.empty:
  df = pd.concat(
  [
  df[non_percent_metric_columns],
  