Symfony version(s) affected: 4.3.2+

Description
Using the !tagged handler in YAML service definitions throws

Notice: Undefined index: index_by

How to reproduce
Define any service with:

App\MyService:
    $tagged: !tagged_iterator {tag: test.tag}

Please note that the string version works:

App\MyService:
    $tagged: !tagged_iterator test.tag

Possible Solution
Although the key seems to be optional (at least all the docs tell me it should be) there is no fallback in the array access.

https://github.com/symfony/symfony/blob/4.4/src/Symfony/Component/DependencyInjection/Loader/YamlFileLoader.php#L744

This line:

$argument = new TaggedIteratorArgument($argument['tag'], $argument['index_by'], $argument['default_index_method'] ?? null, $forLocator);

should probably be

$argument = new TaggedIteratorArgument($argument['tag'], $argument['index_by'] ?? null, $argument['default_index_method'] ?? null, $forLocator);
//                                                                            ^^^^^^^^

Additional context