Fix file version + linux logic for _cached

packages/shared_preferences/shared_preferences_linux/lib/shared_preferences_linux.dart

@@ -40,7 +40,6 @@ class SharedPreferencesLinux extends SharedPreferencesStorePlatform {
       return _cachedPreferences!;
     }
 
-    _cachedPreferences = {};
     var localDataFile = await _getLocalDataFile();
     if (localDataFile.existsSync()) {
       String stringMap = localDataFile.readAsStringSync();
@@ -50,6 +49,7 @@ class SharedPreferencesLinux extends SharedPreferencesStorePlatform {
       }
     }
 
+    _cachedPreferences ??= {};
     return _cachedPreferences!;
   }
 

packages/shared_preferences/shared_preferences_linux/pubspec.yaml

@@ -17,7 +17,7 @@ environment:
 dependencies:
   flutter:
     sdk: flutter
-  file: ">=6.0.0-nullsafety.4 <7.0.0"
+  file: ^6.0.0-nullsafety.4
   meta: ^1.0.4
   path: ^1.8.0-nullsafety.3
   path_provider_linux: ^0.2.0-nullsafety