minor #40484 [DependencyInjection] accept null index in #[TaggedItem] (nicolas-grekas)

This PR was merged into the 5.3-dev branch.

Discussion
----------

[DependencyInjection] accept null index in #[TaggedItem]

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

As hinted by @stof in https://github.com/symfony/symfony/pull/40248#discussion_r595065941

Commits
-------

6d16fac703 [DI] accept null index in #[TaggedItem]

src/Symfony/Component/DependencyInjection/Attribute/TaggedItem.php

@@ -20,7 +20,7 @@ namespace Symfony\Component\DependencyInjection\Attribute;
 class TaggedItem
 {
     public function __construct(
-        public string $index,
+        public ?string $index = null,
         public ?int $priority = null,
     ) {
     }

src/Symfony/Component/DependencyInjection/Tests/Compiler/PriorityTaggedServiceTraitTest.php

@@ -205,6 +205,9 @@ class PriorityTaggedServiceTraitTest extends TestCase
                 HelloNamedService::class => (new ChildDefinition(''))->addTag('my_custom_tag'),
                 \stdClass::class => (new ChildDefinition(''))->addTag('my_custom_tag2'),
             ]);
+        $container->register('service3', HelloNamedService2::class)
+            ->setAutoconfigured(true)
+            ->addTag('my_custom_tag');
 
         (new ResolveInstanceofConditionalsPass())->process($container);
 
@@ -212,6 +215,7 @@ class PriorityTaggedServiceTraitTest extends TestCase
 
         $tag = new TaggedIteratorArgument('my_custom_tag', 'foo', 'getFooBar');
         $expected = [
+            'service3' => new TypedReference('service3', HelloNamedService2::class),
             'hello' => new TypedReference('service2', HelloNamedService::class),
             'service1' => new TypedReference('service1', FooTagClass::class),
         ];
@@ -235,3 +239,8 @@ class PriorityTaggedServiceTraitImplementation
 class HelloNamedService extends \stdClass
 {
 }
+
+#[TaggedItem(priority: 2)]
+class HelloNamedService2
+{
+}