code: def homogenize_types(df, groupby_cols):
          
          for col in groupby_cols:
              df[col] = df[col].fillna('<NULL>').astype('unicode')
          return df
docs: Converting all GROUPBY columns to strings
  
          When grouping by a numeric (say FLOAT) column, pydruid returns
          strings in the dataframe. This creates issues downstream related
          to having mixed types in the dataframe
  
          Here we replace None with <NULL> and make the whole series a
          str instead of an object.