Render empty forwarders for forward zones

A forward zone with no forwarders now renders "forwarders { };", which
BIND accepts and treats as disabling forwarding for the zone.

internal/bind/render.go

@@ -154,9 +154,7 @@ func renderForwardZone(z bindv1alpha1.BindZone, indent string) string {
 	b.WriteString(fmt.Sprintf("%szone \"%s\" {\n", indent, z.Spec.ZoneName))
 	b.WriteString(indent + "    type forward;\n")
 	b.WriteString(indent + "    forward only;\n")
-	if len(z.Spec.Forwarders) > 0 {
-		b.WriteString(fmt.Sprintf("%s    forwarders { %s };\n", indent, terminate(z.Spec.Forwarders)))
-	}
+	b.WriteString(indent + "    " + ForwardersClause(z.Spec.Forwarders) + ";\n")
 	b.WriteString(indent + "};\n")
 	return b.String()
 }
@@ -398,6 +396,14 @@ func terminate(entries []string) string {
 	return strings.Join(parts, " ")
 }
 
+// ForwardersClause renders a zone forwarders clause; empty means "forwarders { }" (no forwarding).
+func ForwardersClause(entries []string) string {
+	if list := terminate(entries); list != "" {
+		return "forwarders { " + list + " }"
+	}
+	return "forwarders { }"
+}
+
 func yesno(b bool) string {
 	if b {
 		return "yes"