move xml serialize_null config into doctype_whitelist config node

DependencyInjection/Configuration.php

@@ -170,13 +170,6 @@ class Configuration implements ConfigurationInterface
                             ->arrayNode('doctype_whitelist')
                                 ->prototype('scalar')->end()
                             ->end()
-                        ->end()
-                    ->end()
-                ->end()
-                ->children()
-                    ->arrayNode('xml')
-                        ->addDefaultsIfNotSet()
-                        ->children()
                             ->booleanNode('serialize_null')->defaultFalse()->end()
                         ->end()
                     ->end()