Don't write compound header to zero length files

Change-Id: Ie964ca8b870714d1c3e92469a771fab9a71f0633
Reviewed-on: https://gerrit.libreoffice.org/8220
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>