message: FilterBox,BigNumber,WorldMap: Handle empty results
  This change avoids Pandas errors to pop up in chart when no data
  is returned (confusing users). In this way a nicer
  "No Results etc.." is returned.
diff: @@ -1143,6 +1143,9 @@ class BigNumberViz(BaseViz):
  return d
  def get_data(self, df: pd.DataFrame) -> VizData:
  + if df.empty:
  + return None
  +
  df = df.pivot_table(
  index=DTTM_ALIAS,
  columns=[],
  @@ -1878,6 +1881,9 @@ class WorldMapViz(BaseViz):
  return qry
  def get_data(self, df: pd.DataFrame) -> VizData:
  + if df.empty:
  + return None
  +
  from superset.examples import countries
  fd = self.form_data
  @@ -1952,7 +1958,7 @@ class FilterBoxViz(BaseViz):
  col = flt.get("column")
  metric = flt.get("metric")
  df = self.dataframes.get(col)
  - if df is not None:
  + if df is not None and not df.empty:
  if metric:
  df = df.sort_values(
  utils.get_metric_name(metric), ascending=flt.get("asc")
  @@ -1967,6 +1973,8 @@ class FilterBoxViz(BaseViz):
  {"id": row[0], "text": row[0]}
  for row in df.itertuples(index=False)
  ]
  + if not d:
  + return None
  return d
  