bug #27472 [DI] Ignore missing tree root nodes on validate (ro0NL)

This PR was squashed before being merged into the 4.1 branch (closes #27472).

Discussion
----------

[DI] Ignore missing tree root nodes on validate

| Q             | A
| ------------- | ---
| Branch?       | 4.1
| Bug fix?      | yes
| New feature?  | technically yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #27450
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

<!--
Write a short README entry for your feature/bugfix here (replace this comment block.)
This will help people understand your PR and can be used as a start of the Doc PR.
Additionally:
 - Bug fixes must be submitted against the lowest branch where they apply
   (lowest branches are regularly merged to upper ones so they get the fixes too).
 - Features and deprecations must be submitted against the master branch.
-->

Commits
-------

b3cdfc6 [DI] Ignore missing tree root nodes on validate

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

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Config\Definition\Builder;
 
+use Symfony\Component\Config\Definition\Exception\TreeWithoutRootNodeException;
 use Symfony\Component\Config\Definition\NodeInterface;
 
 /**
@@ -74,7 +75,7 @@ public function setPathSeparator(string $separator)
     private function assertTreeHasRootNode()
     {
         if (null === $this->root) {
-            throw new \RuntimeException('The configuration tree has no root node.');
+            throw new TreeWithoutRootNodeException('The configuration tree has no root node.');
         }
     }
 }

src/Symfony/Component/Config/Definition/Exception/TreeWithoutRootNodeException.php

@@ -0,0 +1,19 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Config\Definition\Exception;
+
+/**
+ * @author Roland Franssen <franssen.roland@gmail.com>
+ */
+class TreeWithoutRootNodeException extends \RuntimeException
+{
+}

src/Symfony/Component/DependencyInjection/Compiler/ValidateEnvPlaceholdersPass.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\DependencyInjection\Compiler;
 
 use Symfony\Component\Config\Definition\BaseNode;
+use Symfony\Component\Config\Definition\Exception\TreeWithoutRootNodeException;
 use Symfony\Component\Config\Definition\Processor;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Extension\ConfigurationExtensionInterface;
@@ -75,7 +76,10 @@ public function process(ContainerBuilder $container)
                     continue;
                 }
 
-                $processor->processConfiguration($configuration, $config);
+                try {
+                    $processor->processConfiguration($configuration, $config);
+                } catch (TreeWithoutRootNodeException $e) {
+                }
             }
         } finally {
             BaseNode::resetPlaceholders();

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

@@ -14,6 +14,7 @@
 use PHPUnit\Framework\TestCase;
 use Symfony\Component\Config\Definition\Builder\TreeBuilder;
 use Symfony\Component\Config\Definition\ConfigurationInterface;
+use Symfony\Component\Config\Definition\Exception\TreeWithoutRootNodeException;
 use Symfony\Component\DependencyInjection\Compiler\MergeExtensionConfigurationPass;
 use Symfony\Component\DependencyInjection\Compiler\RegisterEnvVarProcessorsPass;
 use Symfony\Component\DependencyInjection\Compiler\ValidateEnvPlaceholdersPass;
@@ -222,6 +223,17 @@ public function testEnvWithVariableNode(): void
         $this->assertSame($expected, $container->resolveEnvPlaceholders($ext->getConfig()));
     }
 
+    public function testConfigurationWithoutRootNode(): void
+    {
+        $container = new ContainerBuilder();
+        $container->registerExtension($ext = new EnvExtension(new EnvConfigurationWithoutRootNode()));
+        $container->loadFromExtension('env_extension');
+
+        $this->doProcess($container);
+
+        $this->addToAssertionCount(1);
+    }
+
     private function doProcess(ContainerBuilder $container): void
     {
         (new MergeExtensionConfigurationPass())->process($container);
@@ -267,23 +279,41 @@ public function getConfigTreeBuilder()
     }
 }
 
+class EnvConfigurationWithoutRootNode implements ConfigurationInterface
+{
+    public function getConfigTreeBuilder()
+    {
+        return new TreeBuilder();
+    }
+}
+
 class EnvExtension extends Extension
 {
+    private $configuration;
     private $config;
 
+    public function __construct(ConfigurationInterface $configuration = null)
+    {
+        $this->configuration = $configuration ?? new EnvConfiguration();
+    }
+
     public function getAlias()
     {
         return 'env_extension';
     }
 
     public function getConfiguration(array $config, ContainerBuilder $container)
     {
-        return new EnvConfiguration();
+        return $this->configuration;
     }
 
     public function load(array $configs, ContainerBuilder $container)
     {
-        $this->config = $this->processConfiguration($this->getConfiguration($configs, $container), $configs);
+        try {
+            $this->config = $this->processConfiguration($this->getConfiguration($configs, $container), $configs);
+        } catch (TreeWithoutRootNodeException $e) {
+            $this->config = null;
+        }
     }
 
     public function getConfig()