plugin/pkg/dnsutil: guard Join against an empty label slice (#8225)

plugin/pkg/dnsutil/join.go

@@ -10,6 +10,9 @@ import (
 // the root label it is ignored. Not other syntax checks are performed.
 func Join(labels ...string) string {
 	ll := len(labels)
+	if ll == 0 {
+		return "."
+	}
 	if labels[ll-1] == "." {
 		return strings.Join(labels[:ll-1], ".") + "."
 	}