Fix for stevvooe:check-storage-drivers-list-path-not-found in OSS driver

Change-Id: I5e96fe761d3833c962084fd2d597f47e8a72e7c2
Signed-off-by: Li Yi <denverdino@gmail.com>

registry/storage/driver/oss/oss.go

@@ -669,6 +669,12 @@ 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 OSS.
+		return nil, storagedriver.PathNotFoundError{Path: path}
+	}
+
 	files := []string{}
 	directories := []string{}