bug #38580 [FrameworkBundle] fix config declaration of http_cache option (nicolas-grekas)

This PR was merged into the 5.x branch.

Discussion
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[FrameworkBundle] fix config declaration of http_cache option

| Q             | A
| ------------- | ---
| Branch?       | 5.2
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Addresses comments from @stof in symfony/symfony#37351 (review)

Commits
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2514cf1c1d [FrameworkBundle] fix config declaration of http_cache option

Author: fabpot

DependencyInjection/Configuration.php

@@ -218,6 +218,7 @@ private function addHttpCacheSection(ArrayNodeDefinition $rootNode)
                 ->arrayNode('http_cache')
                     ->info('HTTP cache configuration')
                     ->canBeEnabled()
+                    ->fixXmlConfig('private_header')
                     ->children()
                         ->booleanNode('debug')->defaultValue('%kernel.debug%')->end()
                         ->enumNode('trace_level')
@@ -227,8 +228,6 @@ private function addHttpCacheSection(ArrayNodeDefinition $rootNode)
                         ->integerNode('default_ttl')->end()
                         ->arrayNode('private_headers')
                             ->performNoDeepMerging()
-                            ->requiresAtLeastOneElement()
-                            ->fixXmlConfig('private_header')
                             ->scalarPrototype()->end()
                         ->end()
                         ->booleanNode('allow_reload')->end()