bug #27710 [DependencyInjection] fix handling of empty DI extension configs (xabbuh)

This PR was merged into the 4.1 branch.

Discussion
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[DependencyInjection] fix handling of empty DI extension configs

| Q             | A
| ------------- | ---
| Branch?       | 4.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #27707
| License       | MIT
| Doc PR        |

Commits
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05d4bcb987 fix handling of empty DI extension configs

Author: nicolas-grekas

Compiler/ValidateEnvPlaceholdersPass.php

@@ -65,7 +65,7 @@ public function process(ContainerBuilder $container)
             $processor = new Processor();
 
             foreach ($extensions as $name => $extension) {
-                if (!$extension instanceof ConfigurationExtensionInterface || !$config = $container->getExtensionConfig($name)) {
+                if (!$extension instanceof ConfigurationExtensionInterface || !$config = array_filter($container->getExtensionConfig($name))) {
                     // this extension has no semantic configuration or was not called
                     continue;
                 }