fix: dev: Avoid named assertion failure during parent-NS lookups when none exist

Configuring the root zone as a signed primary with parental agents (or with
notify-on-cds-changes) caused named to exit on an internal assertion as soon
as the DS-publication machinery tried to look up the parent NS RRset — the root
has no parent. The lookup is now short-circuited cleanly.

Similar, a zone with no NS records in the parent caused named to exit in the same way.

Closes #5910

Merge branch '5910-nsfetch-start-root-domain-assertion' into 'main'

See merge request isc-projects/bind9!11909