Respond TXT record even if TXT field is nil

[yuuzi41]

Hello,

It responds TXT Record even if the TXT field of MDNSService is nil here https://github.com/hashicorp/mdns/blob/main/zone.go#L305
But I think it'd be better if it responded nothing when it received TXT query and the TXT field is nil.

Any suggestions or thoughts would be appreciated.