message: [examples] Fix, sf population polygons div by zero error
diff: import json
  import pandas as pd
  -from sqlalchemy import BigInteger, Text
  +from sqlalchemy import BigInteger, Float, Text
  from superset import db
  from superset.utils import core as utils
  @@ -43,7 +43,7 @@ def load_sf_population_polygons(only_metadata=False, force=False):
  "zipcode": BigInteger,
  "population": BigInteger,
  "contour": Text,
  - "area": BigInteger,
  + "area": Float,
  },
  index=False,
  )
  