bug #20341 Fix YamlReferenceDumper unnamed nested prototypes (ogizanagi)

This PR was squashed before being merged into the 3.2-dev branch (closes #20341).

Discussion
----------

Fix YamlReferenceDumper unnamed nested prototypes

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony/symfony#20340
| License       | MIT
| Doc PR        | N/A

Ideally, I'd like to output something like:

```yml
pipou:

    # Prototype
    name:

        # Prototype
        -
            didou:                ~
```

But for now, this should fix the regression.

Commits
-------

8e04643 Fix YamlReferenceDumper unnamed nested prototypes

Author: fabpot

Definition/Dumper/YamlReferenceDumper.php

@@ -201,7 +201,7 @@ private function getPrototypeChildren(PrototypedArrayNode $node)
             $keyNode = new ArrayNode($key, $node);
             $children = $prototype->getChildren();
 
-            if ($prototype instanceof PrototypedArrayNode) {
+            if ($prototype instanceof PrototypedArrayNode && $prototype->getKeyAttribute()) {
                 $children = $this->getPrototypeChildren($prototype);
             }
 

Tests/Definition/Dumper/XmlReferenceDumperTest.php

@@ -92,6 +92,14 @@ enum=""
 
     </cms-page>
 
+    <!-- prototype -->
+    <pipou name="pipou name">
+
+        <!-- prototype -->
+        <name didou="" />
+
+    </pipou>
+
 </config>
 
 EOL

Tests/Definition/Dumper/YamlReferenceDumperTest.php

@@ -74,6 +74,10 @@ enum:                 ~ # One of "this"; "that"
             locale:
                 title:                ~ # Required
                 path:                 ~ # Required
+    pipou:
+
+        # Prototype
+        name:                 []
 
 EOL;
     }

Tests/Fixtures/Configuration/ExampleConfiguration.php

@@ -80,6 +80,17 @@ public function getConfigTreeBuilder()
                         ->end()
                     ->end()
                 ->end()
+                ->arrayNode('pipou')
+                    ->useAttributeAsKey('name')
+                    ->prototype('array')
+                        ->prototype('array')
+                            ->children()
+                                ->scalarNode('didou')
+                                ->end()
+                            ->end()
+                        ->end()
+                    ->end()
+                ->end()
             ->end()
         ;