v2.1.1

• Change the default value of na_values in PandasCursor from None to empty character.
  If you want to change the behavior to that of the previous version, explicitly set na_values to None in the argument of the execute method.

  import pyathena
  from pyathena.pandas.cursor import PandasCursor
  cursor = pyathena.connect(s3_staging_dir="s3://YOUR_S3_BUCKET/path/to/",
                            region_name="us-west-2",
                            cursor_class=PandasCursor).cursor()
  df = cursor.execute("SELECT * FROM many_rows",
                      na_values=None).as_pandas()
  

  Thanks @EdwardJRoss. (#204)