fix: address code review issues on PR #9

- gofmt main.go to fix tab indentation in main() body
- stats.go ComputeSummary: return nil when Answers and ByType are empty
- dns/types.go: remove duplicate ParseQueryType (dead code)
- formatter.go: log hook errors to stderr when Debug > 0; add Debug to Config
- text.go referralID: remove redundant depth==0 branch
- text.go writeServers: remove version lookup placeholder string

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/output/stats.go

@@ -73,6 +73,9 @@ func ComputeSummary(results []traverse.TraversalResult) *SummaryStats {
 		return stats.Answers[i].RData < stats.Answers[j].RData
 	})
 
+	if len(stats.Answers) == 0 && len(stats.ByType) == 0 {
+		return nil
+	}
 	return stats
 }