When a secondary received an IXFR that transitioned a zone from unsigned to NSEC-signed, queries for empty non-terminal names returned the zone apex NSEC record instead of the NSEC that actually covers the queried name. The issue only occurred with incremental transfers; a full AXFR or a server restart resolved it.
Closes #5824
Merge branch 'ondrej/fix-nsec-ixfr' into 'main'
See merge request isc-projects/bind9!11786