Handle `S3_BUCKET` being an empty string by default now.
Handle `S3_BUCKET` being an empty string by default now.

This was introduced with
https://github.com/mesosphere/dcos-commons/commit/31224ec8dcddc1e5f4f090acfc38f8c6f7e275e5

`dict.get` does not return the default if the value is an empty string so
we need to check for that.

mesosphere/dcos-commons,mesosphere/dcos-commons,vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,vishnu2kmohan/dcos-commons,mesosphere/dcos-commons