enabled null safety for files which are already null safe

lib/core/cache/thumbnail_cache_manager.dart

@@ -1,4 +1,4 @@
-// @dart=2.9
+
 
 import 'package:flutter_cache_manager/flutter_cache_manager.dart';
 

lib/core/cache/video_cache_manager.dart

@@ -1,4 +1,4 @@
-// @dart=2.9
+
 
 import 'package:flutter_cache_manager/flutter_cache_manager.dart';
 

lib/core/constants.dart

@@ -1,4 +1,4 @@
-// @dart=2.9
+
 
 const int kThumbnailSmallSize = 256;
 const int kThumbnailQuality = 50;

lib/core/event_bus.dart

@@ -1,4 +1,4 @@
-// @dart=2.9
+
 
 import 'package:event_bus/event_bus.dart';
 

lib/models/update_share_url_request.dart

@@ -1,3 +0,0 @@
-// @dart=2.9
-
-