(bug): Ensure Contributions\Repository handles faulty $rows values

Sentry Issue: ENGINE-J

Error: Call to a member function pagingStateToken() on boolean
  File "/Core/Rewards/Contributions/Repository.php", line 124, in Minds\Core\Rewards\Contributions\Repository::getList
    'token' => $rows->pagingStateToken()
  File "/Controllers/api/v2/blockchain/contributions.php", line 36, in Minds\Controllers\api\v2\blockchain\contributions::get
    'offset' => $offset
  File "/Api/Factory.php", line 69, in Minds\Api\Factory::build
    return $handler->$method($pages);
  File "/Controllers/api/api.php", line 76, in Minds\Controllers\api\api::get
    return Factory::build($pages, $this->request, $this->response);
  File "/Core/Router.php", line 137, in Minds\Core\Router::route
    return $handler->$method($pages);
...
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