s3: ignore directory markers at the root also

See: https://forum.rclone.org/t/issue-with-lsf-r-files-only-first-line-is-blank/15229/

backend/s3/s3.go

@@ -1464,7 +1464,7 @@ func (f *Fs) list(ctx context.Context, bucket, directory, prefix string, addBuck
 				continue
 			}
 			remote = remote[len(prefix):]
-			isDirectory := strings.HasSuffix(remote, "/")
+			isDirectory := remote == "" || strings.HasSuffix(remote, "/")
 			if addBucket {
 				remote = path.Join(bucket, remote)
 			}