* Fix to make it None when SQLAlchemy's URI username and password are empty strings.
If you do not specify `aws_access_key_id` and `aws_secret_access_key` in SQLAlchemy using the instance profile or boto3 configuration file, the connection string is as follows:
awsathena+rest://:athena.{region_name}.amazonaws.com:443/{schema_name}?s3_staging_dir={s3_staging_dir}&...
Thanks leahein. (laughingman7743/PyAthena/issues/54)
* Fix double escaping of percent character in SQLAlchemy.
Thanks mister-average. (laughingman7743/PyAthena/issues/56)