When a given prefix is empty and we attempt to list its content AWS
returns that the prefix contains one object with key defined as the
prefix with an extra "/" at the end.
e.g.
If we call ListObjects() passing to it an existing but empty prefix,
say "my/empty/prefix", AWS will return that "my/empty/prefix/" is an
object inside "my/empty/prefix" (ListObjectsOutput.Contents).
This extra "/" causes the upload purging process to panic. On normal
circunstances we never find empty prefixes on S3 but users may touch
it.
Signed-off-by: Ricardo Maraschini <rmarasch@redhat.com>