knotc zone-flush doesn't flush zone when zone is not signed.

With default zonefile-sync: 0 knotc zone-flush only flush dnssec signed zone to disk.

knotc --force zone-flush does sync zone to disk but it's not expected to need --force with zonefile-sync: 0 default setting and when dnssec signed zone doesn't need --force.

Same issue with knotc zone-flush even with zonemd-generate: zonemd-sha384 when zone is unsigned.