storage/driver/s3: correct response on list of missing directory

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/driver/s3/s3.go

@@ -685,6 +685,12 @@ func (d *driver) List(ctx context.Context, path string) ([]string, error) {
 		return nil, err
 	}
 
+	if len(listResponse.Contents) == 0 {
+		// Treat empty response as missing directory, since we don't actually
+		// have directories in s3.
+		return nil, storagedriver.PathNotFoundError{Path: path}
+	}
+
 	files := []string{}
 	directories := []string{}