Don't create a directory when there is no report prefix.

[blentz]

When no report prefix is given, nise still creates a prefix directory instead of uploading the report one level higher in the tree.

Example:

$ nise --start-date 06-01-2018 --end-date 02-01-2019 --aws --aws-s3-bucket-name blentz-testing --aws-s3-report-name blentz-test 
$ aws s3 ls blentz-testing
                           PRE /
 $ aws s3 ls blentz-testing/
                           PRE /
 $ aws s3 ls blentz-testing//
                           PRE blentz-test/
 $ aws s3 ls blentz-testing//blentz-test
                           PRE blentz-test/
 $ aws s3 ls blentz-testing//blentz-test/
                           PRE 20180601-20180701/
                           PRE 20180701-20180801/
                           PRE 20180801-20180901/
                           PRE 20180901-20181001/
                           PRE 20181001-20181101/
                           PRE 20181101-20181201/
                           PRE 20181201-20190101/
                           PRE 20190101-20190201/
                           PRE 20190201-20190301/

The path should not be blentz-testing//blentz-test/ it should instead be blentz-testing/blentz-test/