Add tests for null no-change and zero as real value

desktop-client/tests/state.test.js

@@ -45,3 +45,15 @@ test('mergeState does not mutate inputs', () => {
   assert.deepEqual(current, before);
   assert.deepEqual(update, { title: 'X' });
 });
+
+test('mergeState treats null fields as no-change', () => {
+  const current = { ...initialState, title: 'Previous' };
+  const next = mergeState(current, { title: null });
+  assert.equal(next.title, 'Previous');
+});
+
+test('mergeState applies 0 as a real value (not a no-change)', () => {
+  const current = { ...initialState, progress: 42 };
+  const next = mergeState(current, { progress: 0 });
+  assert.equal(next.progress, 0);
+});