nsec3: resalt to empty salt if reconfigured salt size to 0

src/knot/dnssec/kasp/kasp_db.c

src/knot/dnssec/kasp/kasp_zone.c

src/knot/dnssec/zone-events.c

tests-extra/tests/dnssec/no_resign/test.py

tests-extra/tests/dnssec/nsec_changes/test.py