migrated image_cache.dart to null safety

lib/core/cache/image_cache.dart

@@ -1,5 +1,3 @@
-// @dart=2.9
-
 import 'dart:io' as io;
 
 import 'package:photos/core/cache/lru_map.dart';
@@ -7,11 +5,11 @@ import 'package:photos/core/cache/lru_map.dart';
 class FileLruCache {
   static final LRUMap<String, io.File> _map = LRUMap(25);
 
-  static io.File get(String key) {
+  static io.File? get(String key) {
     return _map.get(key);
   }
 
-  static void put(String key, io.File imageData) {
-    _map.put(key, imageData);
+  static void put(String key, io.File value) {
+    _map.put(key, value);
   }
 }