message: [druid] fix bug around handling NULLs
  fillna would miss out on identifying STRING columns for Druid and
  replace None in string columns with a numeric `0`. This
  mixed type column would confuse
  pandas down the line on some operations like `df.pivot_table`.
diff: @@ -68,10 +68,10 @@ class BaseViz(object):
  self.status = None
  self.error_message = None
  - def get_fillna_for_type(self, col_type):
  + def get_fillna_for_col(self, col):
  """Returns the value for use as filler for a specific Column.type"""
  - if col_type:
  - if col_type == 'TEXT' or col_type.startswith('VARCHAR'):
  + if col:
  + if col.is_string:
  return ' NULL'
  return self.default_fillna
  @@ -79,8 +79,11 @@ class BaseViz(object):
  """Returns a dict or scalar that can be passed to DataFrame.fillna"""
  if columns is None:
  return self.default_fillna
  - columns_types = self.datasource.columns_types
  - fillna = {c: self.get_fillna_for_type(columns_types.get(c)) for c in columns}
  + columns_dict = {col.column_name: col for col in self.datasource.columns}
  + fillna = {
  + c: self.get_fillna_for_col(columns_dict.get(c))
  + for c in columns
  + }
  return fillna
  def get_df(self, query_obj=None):
  