bug #29152 [Config] Unset key during normalization (ro0NL)

This PR was squashed before being merged into the 2.8 branch (closes #29152).

Discussion
----------

[Config] Unset key during normalization

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes-ish
| New feature?  | yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #29142
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

2.8 vs. 4.x :) let me know.

Commits
-------

e1402d495e [Config] Unset key during normalization

src/Symfony/Component/Config/Definition/ArrayNode.php

@@ -288,7 +288,10 @@ class ArrayNode extends BaseNode implements PrototypeNodeInterface
         $normalized = array();
         foreach ($value as $name => $val) {
             if (isset($this->children[$name])) {
-                $normalized[$name] = $this->children[$name]->normalize($val);
+                try {
+                    $normalized[$name] = $this->children[$name]->normalize($val);
+                } catch (UnsetKeyException $e) {
+                }
                 unset($value[$name]);
             } elseif (!$this->removeExtraKeys) {
                 $normalized[$name] = $val;

src/Symfony/Component/Config/Definition/Builder/ExprBuilder.php

@@ -149,7 +149,7 @@ class ExprBuilder
     }
 
     /**
-     * Sets a closure marking the value as invalid at validation time.
+     * Sets a closure marking the value as invalid at processing time.
      *
      * if you want to add the value of the node in your message just use a %s placeholder.
      *
@@ -167,7 +167,7 @@ class ExprBuilder
     }
 
     /**
-     * Sets a closure unsetting this key of the array at validation time.
+     * Sets a closure unsetting this key of the array at processing time.
      *
      * @return $this
      *

src/Symfony/Component/Config/Tests/Definition/Builder/ArrayNodeDefinitionTest.php

@@ -231,6 +231,25 @@ class ArrayNodeDefinitionTest extends TestCase
         $this->assertFalse($this->getField($node, 'normalizeKeys'));
     }
 
+    public function testUnsetChild()
+    {
+        $node = new ArrayNodeDefinition('root');
+        $node
+            ->children()
+                ->scalarNode('value')
+                    ->beforeNormalization()
+                        ->ifTrue(function ($value) {
+                            return empty($value);
+                        })
+                        ->thenUnset()
+                    ->end()
+                ->end()
+            ->end()
+        ;
+
+        $this->assertSame(array(), $node->getNode()->normalize(array('value' => null)));
+    }
+
     public function getEnableableNodeFixtures()
     {
         return array(