fix s3 doesDirectoryExist check for empty directories

fix s3 doesDirectoryExist check for empty directories

if a directory is empty, only the 'marker' object `/` exists.

since not all s3 implementations return just the prefix when listing objects by prefix,
when listing objects by the folder prefix, nothing will be returned if the directory is empty.
by not including the trailing slash in the prefix, the folder marked will always be returned if it exists

Signed-off-by: Robin Appelman <robin@icewind.nl>