migrate collection_items.dart to null safety

lib/models/collection_items.dart

@@ -1,11 +1,9 @@
-// @dart=2.9
-
 import 'package:photos/models/collection.dart';
 import 'package:photos/models/file.dart';
 
 class CollectionWithThumbnail {
   final Collection collection;
-  final File thumbnail;
+  final File? thumbnail;
 
   CollectionWithThumbnail(
     this.collection,

lib/models/search/album_search_result.dart

@@ -19,7 +19,7 @@ class AlbumSearchResult extends SearchResult {
 
   @override
   File previewThumbnail() {
-    return collectionWithThumbnail.thumbnail;
+    return collectionWithThumbnail.thumbnail!;
   }
 
   @override