fix: without api_platform.defaults config key, DI extension fail

Error during config DI processing 

```sh
compose --env-file .env-docker run --rm -t  php-cli php bin/console deb:conf api_platform 

In ApiPlatformExtension.php line 312:
                                           
  Warning: Undefined array key "defaults"
```

Author: chadyred

src/Symfony/Bundle/DependencyInjection/ApiPlatformExtension.php

@@ -309,7 +309,7 @@ private function registerCommonConfiguration(ContainerBuilder $container, array
         $container->setParameter('api_platform.collection.pagination.client_items_per_page', $config['defaults']['pagination_client_items_per_page'] ?? false);
         $container->setParameter('api_platform.collection.pagination.client_partial', $config['defaults']['pagination_client_partial'] ?? false);
         $container->setParameter('api_platform.collection.pagination.items_per_page', $config['defaults']['pagination_items_per_page'] ?? 30);
-        $container->setParameter('api_platform.collection.pagination.maximum_items_per_page', \array_key_exists('pagination_maximum_items_per_page', $config['defaults']) ? $config['defaults']['pagination_maximum_items_per_page'] : 30);
+        $container->setParameter('api_platform.collection.pagination.maximum_items_per_page', $config['defaults']['pagination_maximum_items_per_page'] ?? 30);
         $container->setParameter('api_platform.collection.pagination.page_parameter_name', $config['defaults']['pagination_page_parameter_name'] ?? $config['collection']['pagination']['page_parameter_name']);
         $container->setParameter('api_platform.collection.pagination.enabled_parameter_name', $config['defaults']['pagination_enabled_parameter_name'] ?? $config['collection']['pagination']['enabled_parameter_name']);
         $container->setParameter('api_platform.collection.pagination.items_per_page_parameter_name', $config['defaults']['pagination_items_per_page_parameter_name'] ?? $config['collection']['pagination']['items_per_page_parameter_name']);