fix: empty changeset must serialize as [] not null (white-screen after snapshot)

toChangesetResponse initialises updates/prunes/readOnly so a zone matching
its template exactly (e.g. right after 'create template from zone') marshals
arrays, not null. DiffView/DomainDiffPage also normalise null defensively.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwxdSt4reTm7Dj1oxRvpP3

internal/api/api_test.go

@@ -136,3 +136,20 @@ func TestApplyBadUUID(t *testing.T) {
 		t.Fatalf("expected 400 for bad uuid, got %d", w.Code)
 	}
 }
+
+// TestChangesetResponseEmptyMarshalsToArrays guards the белый-экран bug: an
+// empty changeset (zone matches its template exactly, e.g. right after a
+// snapshot) must marshal updates/prunes/readOnly as [] not null — a nil slice
+// becomes JSON null and crashes the client's .length/.map calls.
+func TestChangesetResponseEmptyMarshalsToArrays(t *testing.T) {
+	b, err := json.Marshal(toChangesetResponse(diff.Changeset{}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	s := string(b)
+	for _, want := range []string{`"updates":[]`, `"prunes":[]`, `"readOnly":[]`} {
+		if !strings.Contains(s, want) {
+			t.Fatalf("expected %s in %s", want, s)
+		}
+	}
+}