[9.20] fix: usr: Fix nxdomain-redirect combined with dns64

When a resolver was configured with both `nxdomain-redirect` and `dns64`
in the same view, an AAAA query for a nonexistent name could abort
`named`. The combination failed whenever the redirect zone held A
records but no AAAA records.  The server now serves the empty AAAA
response from the redirect zone as-is, instead of attempting DNS64
synthesis on top of it.

Closes #5789

Backport of MR !12059

Merge branch 'backport-5789-fix-nxdomain-redirect-dns64-assert-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12123