migrated thumbnail_cache.dart to null safety

lib/core/cache/thumbnail_cache.dart

@@ -1,5 +1,3 @@
-// @dart=2.9
-
 import 'dart:typed_data';
 
 import 'package:photos/core/cache/lru_map.dart';
@@ -7,9 +5,9 @@ import 'package:photos/core/constants.dart';
 import 'package:photos/models/ente_file.dart';
 
 class ThumbnailLruCache {
-  static final LRUMap<String, Uint8List> _map = LRUMap(1000);
+  static final LRUMap<String, Uint8List?> _map = LRUMap(1000);
 
-  static Uint8List get(EnteFile enteFile, [int size]) {
+  static Uint8List? get(EnteFile enteFile, [int? size]) {
     return _map.get(
       enteFile.cacheKey() +
           "_" +
@@ -19,8 +17,8 @@ class ThumbnailLruCache {
 
   static void put(
     EnteFile enteFile,
-    Uint8List imageData, [
-    int size,
+    Uint8List? imageData, [
+    int? size,
   ]) {
     _map.put(
       enteFile.cacheKey() +