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/PaymentsUserDataProvider.php

@@ -20,9 +20,9 @@ public static function identifier(): string
  return 'payments';
  }
 
- public function data($userId)
+ public function data($userId): ?array
  {
- return [];
+ return null;
  }
 
  public function download($userId)

src/Models/User/RecurrentPaymentsUserDataProvider.php

@@ -19,9 +19,9 @@ public static function identifier(): string
  return 'recurrent_payments';
  }
 
- public function data($userId)
+ public function data($userId): ?array
  {
- return [];
+ return null;
  }
 
  public function download($userId)