resolve: synthesize empty domain only when A and/or AAAA key is requested

resolve: synthesize empty domain only when A and/or AAAA key is requested

Follow-up for 3b2ac14ac45bef01cf489c3231b868936866444b (#22231).

Before this commit.
---
$ dig -t SRV '.'

; <<>> DiG 9.16.24-RH <<>> -t SRV .
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16836
;; flags: qr aa rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;.				IN	SRV

;; Query time: 1 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Fri Feb 04 12:01:09 JST 2022
;; MSG SIZE  rcvd: 28
---

After this commit.
---
$ dig -t SRV '.'

; <<>> DiG 9.16.24-RH <<>> -t SRV .
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 19861
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;.				IN	SRV

;; AUTHORITY SECTION:
.			86394	IN	SOA	a.root-servers.net. nstld.verisign-grs.com. 2022020302 1800 900 604800 86400

;; Query time: 20 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Fri Feb 04 12:00:12 JST 2022
;; MSG SIZE  rcvd: 103
---

Fixes #22401.