Support NODATA (NSEC/NSEC3 use case)

[xinbenlv]

For use case of NSEC/NSEC3 it seems a requirement to support NODATA, and this repo doesn't yet.

See the latest RFC 9077 it reads

[[RFC2308](https://datatracker.ietf.org/doc/html/rfc2308)] defines the TTL of the
Start of Authority (SOA) record that must be returned in negative answers
(NXDOMAIN or NODATA):

Case:

We are trying to deny existence of MX records or TXT records of a domain and only to found that CoreDNS doesn't support NODATA. After digging, I found that it's due to the dependency of this repo from

See

• The line where CoreDNS parses StringToRcode
• The line CoreDNS parsing StringToRcode depends on miekg/dns

Previously the request within this repo to support NODATA is in #428

[Chreo]

I trust you are aware that "NODATA" is not a defined RCODE (https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-6)
When we say "NODATA" that is pseudo for NOERROR RCODE with an empty ANSWER section
That said, it could be potentially useful to have a helper function for DNS messages being defined as:
func (m Msg) IsNODATAResponse() bool
but as with anything, code talks...