Avoid registering the strategy listener when it is useless

If both the client and the async client are null, the strategy would never
find any candidates. In such case, it is useless to register it as a
kernel.request listener to prepend itself to the discovery.

Author: stof

DependencyInjection/HttplugExtension.php

@@ -411,6 +411,12 @@ private function configureAutoDiscoveryClients(ContainerBuilder $container, arra
             $asyncHttpClient = new Reference($asyncHttpClient);
         }
 
+        if (null === $httpClient && null === $asyncHttpClient) {
+            $container->removeDefinition('httplug.strategy');
+
+            return;
+        }
+
         $container
             ->getDefinition('httplug.strategy')
             ->addArgument($httpClient)