fdo#80755 Don't generate thumbnails for encrypted files

(cherry picked from commit a75e0f8e4e5f0baa5805d01c5f8edc7b40fceb0f)

Change-Id: If52532b8e8ca38e82fd95c13080d521d9cb529aa
Reviewed-on: https://gerrit.libreoffice.org/11120
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>