migrated ente_file.dart to null safety

lib/models/ente_file.dart

@@ -1,14 +1,12 @@
 // EnteFile is base file entry for various type of files
 // like DeviceFile,RemoteFile or TrashedFile
 
-// @dart=2.9
-
 abstract class EnteFile {
   // returns cacheKey which should be used while caching entry related to
   // this file.
-  String cacheKey();
+  String? cacheKey();
 
   // returns localIdentifier for the file on the host OS.
   // Can be null if the file only exist on remote
-  String localIdentifier();
+  String? localIdentifier();
 }