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

web/src/components/DiffView.test.tsx

@@ -26,3 +26,17 @@ test("marks read-only records", () => {
   render(<DiffView changeset={cs} />)
   expect(screen.getByText(/NS/)).toBeInTheDocument()
 })
+
+test("does not crash when changeset fields are null", () => {
+  // An empty changeset from an older/edge backend can arrive with null slices
+  // instead of []. DiffView must normalise them, not blow up on .length/.map.
+  const nullish = {
+    updates: null,
+    prunes: null,
+    readOnly: null,
+    inSyncCount: 5,
+  } as unknown as ChangesetResponse
+  render(<DiffView changeset={nullish} />)
+  expect(screen.getByText(/5/)).toBeInTheDocument()
+  expect(screen.getByText(/in sync/)).toBeInTheDocument()
+})