SFTP should accept empty owner/group value when setting attributes

According to SFTP version 6 - section 7.5:

If either the owner or group field is zero length, the field should be considered absent, and no change should be made to that specific field during a modification operation.

see also https://stackoverflow.com/questions/49129859/apache-mina-sshd-project-throws-error-if-file-owner-and-group-owner-is-blank