[Config] Merge array with ignored keys

**Symfony version(s) affected**: 4.3.3

**Description**  
When merging configurations with ` ignoreExtraKeys(false)` configured, the resulting ArrayNode throws a `RuntimeException` => `merge() expects a normalized config array`.

**How to reproduce**  
[Run `run.php`.](https://github.com/alex-dev/symfony-config-merge)
