Added return type ?array to UserDataProviderInterface::data() method

The change should make cached data smaller since a lot of data providers
don't need to provide any user data, but they were still cached as an
empty array into the user data cache (Redis).

remp/crm#2460

src/Models/User/AppleAppstoreUserDataProvider.php

@@ -60,9 +60,9 @@ public static function identifier(): string
         return 'apple_appstore';
     }
 
-    public function data($userId)
+    public function data($userId): ?array
     {
-        return [];
+        return null;
     }
 
     public function download($userId)