A NULL key is a key that has no associated data:
	delegation.secret-wg.org  NS ns.delegation.secret-wg.org                           KEY    49408 3 3                           SIG     KEY É
	ns.delegation.secret-wg.org A 10.1.1.3
	;; No signature over the A RR, it is glue
	;; no signature over the NS: zone is not authoritative for the ;; NS record. It is only a referral