[TwigBundle] Tweak cache warmer configuration

src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php

@@ -25,7 +25,7 @@ class Configuration implements ConfigurationInterface
 
         $rootNode
             ->children()
-                ->scalarNode('cache_warmer')->end()
+                ->scalarNode('cache_warmer')->defaultFalse()->end()
             ->end();
         ;
 

src/Symfony/Bundle/TwigBundle/DependencyInjection/TwigExtension.php

@@ -60,7 +60,7 @@ class TwigExtension extends Extension
             }
         }
 
-        if (!empty($config['cache_warmer'])) {
+        if ($config['cache_warmer']) {
             $container->getDefinition('twig.cache_warmer')->addTag('kernel.cache_warmer');
         }