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/controller/bindzone_notify_test.go

@@ -63,3 +63,23 @@ func TestBuildZoneConfigPrimaryNoNotifyTargets(t *testing.T) {
 		t.Errorf("did not expect notify clauses with no targets: %q", cfg)
 	}
 }
+
+func TestBuildZoneConfigForwardForwarders(t *testing.T) {
+	r := &BindZoneReconciler{}
+	for _, tc := range []struct {
+		fwd  []string
+		want string
+	}{
+		{nil, "{ type forward; forward only; forwarders { }; };"},
+		{[]string{"198.18.19.15"}, "{ type forward; forward only; forwarders { 198.18.19.15; }; };"},
+	} {
+		zone := &bindv1alpha1.BindZone{Spec: bindv1alpha1.BindZoneSpec{ZoneName: "spamhaus.org", Type: bindv1alpha1.ZoneForward, Forwarders: tc.fwd}}
+		cfg, err := r.buildZoneConfig(context.Background(), zone, "", nil, "")
+		if err != nil {
+			t.Fatalf("buildZoneConfig: %v", err)
+		}
+		if cfg != tc.want {
+			t.Errorf("forwarders %v:\ngot  %q\nwant %q", tc.fwd, cfg, tc.want)
+		}
+	}
+}