add proper defaults

Author: mtarld

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

@@ -2404,6 +2404,7 @@ private function addSerDesSection(ArrayNodeDefinition $rootNode, callable $enabl
                             ->info('Defines where to find serializable classes.')
                             ->beforeNormalization()->ifString()->then(function ($v) { return [$v]; })->end()
                             ->prototype('scalar')->end()
+                            ->defaultValue([])
                         ->end()
                         ->arrayNode('template_warm_up')
                             ->addDefaultsIfNotSet()
@@ -2413,6 +2414,7 @@ private function addSerDesSection(ArrayNodeDefinition $rootNode, callable $enabl
                                     ->info('Defines the formats that will be handled.')
                                     ->beforeNormalization()->ifString()->then(function ($v) { return [$v]; })->end()
                                     ->prototype('scalar')->end()
+                                    ->defaultValue(['json'])
                                 ->end()
                             ->end()
                         ->end()