Respond TXT record even if TXT field is nil

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.

Start at zone.go around line 305 and trace how TXT queries are handled when MDNSService.TXT is nil. Confirm the current response behavior, then make the behavior match the issue's requested outcome: no response to a TXT query when the TXT field is nil.

Written by the indexing model from the issue text.