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_test.go

@@ -261,3 +261,19 @@ func TestRenderDeterministicWithShuffledForwards(t *testing.T) {
 		t.Fatalf("render must be independent of forward-zone input order:\n--- A ---\n%s\n--- B ---\n%s", pa, pb)
 	}
 }
+
+func TestRenderForwardZoneForwarders(t *testing.T) {
+	for _, tc := range []struct {
+		fwd  []string
+		want string
+	}{
+		{nil, "    forwarders { };\n"},
+		{[]string{"198.18.19.15", "198.18.19.16;"}, "    forwarders { 198.18.19.15; 198.18.19.16; };\n"},
+	} {
+		z := bindv1alpha1.BindZone{Spec: bindv1alpha1.BindZoneSpec{ZoneName: "spamhaus.org", Type: bindv1alpha1.ZoneForward, Forwarders: tc.fwd}}
+		want := "zone \"spamhaus.org\" {\n    type forward;\n    forward only;\n" + tc.want + "};\n"
+		if got := renderForwardZone(z, ""); got != want {
+			t.Errorf("forwarders %v:\ngot  %q\nwant %q", tc.fwd, got, want)
+		}
+	}
+}