fix: local date time group fall back (#26110)

* fix: local date time group fall back

* remove else clause

mobile/test/infrastructure/repositories/merged_asset_drift_test.dart

@@ -0,0 +1,51 @@
+import 'package:drift/drift.dart';
+import 'package:drift/native.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:immich_mobile/domain/models/asset/base_asset.model.dart';
+import 'package:immich_mobile/domain/models/timeline.model.dart';
+import 'package:immich_mobile/infrastructure/entities/remote_asset.entity.drift.dart';
+import 'package:immich_mobile/infrastructure/entities/user.entity.drift.dart';
+import 'package:immich_mobile/infrastructure/repositories/db.repository.dart';
+
+void main() {
+  late Drift db;
+
+  setUp(() {
+    db = Drift(DatabaseConnection(NativeDatabase.memory(), closeStreamsSynchronously: true));
+  });
+
+  tearDown(() async {
+    await db.close();
+  });
+
+  test('mergedBucket falls back to createdAt when localDateTime is null', () async {
+    const userId = 'user-1';
+    final createdAt = DateTime(2024, 1, 1, 12);
+
+    await db
+        .into(db.userEntity)
+        .insert(UserEntityCompanion.insert(id: userId, email: 'user-1@test.dev', name: 'User 1'));
+
+    await db
+        .into(db.remoteAssetEntity)
+        .insert(
+          RemoteAssetEntityCompanion.insert(
+            id: 'asset-1',
+            name: 'asset-1.jpg',
+            type: AssetType.image,
+            checksum: 'checksum-1',
+            ownerId: userId,
+            visibility: AssetVisibility.timeline,
+            createdAt: Value(createdAt),
+            updatedAt: Value(createdAt),
+            localDateTime: const Value(null),
+          ),
+        );
+
+    final buckets = await db.mergedAssetDrift.mergedBucket(groupBy: GroupAssetsBy.day.index, userIds: [userId]).get();
+
+    expect(buckets, hasLength(1));
+    expect(buckets.single.assetCount, 1);
+    expect(buckets.single.bucketDate, isNotEmpty);
+  });
+}