bug #28861 [DependencyInjection] Skip empty proxy code (olvlvl)

This PR was merged into the 2.8 branch.

Discussion
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[DependencyInjection] Skip empty proxy code

| Q             | A
| ------------- | ---
| Branch?       | 4.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #28852
| License       | MIT

Fix #28852

@nicolas-grekas I'm not sure which branch this should be applied to, please let me know.

Commits
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baf6f8c Skip empty proxy code

src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php

@@ -233,7 +233,9 @@ private function addProxyClasses()
         $strip = '' === $this->docStar && method_exists('Symfony\Component\HttpKernel\Kernel', 'stripComments');
 
         foreach ($definitions as $definition) {
-            $proxyCode = "\n".$this->getProxyDumper()->getProxyCode($definition);
+            if ("\n" === $proxyCode = "\n".$this->getProxyDumper()->getProxyCode($definition)) {
+                continue;
+            }
             if ($strip) {
                 $proxyCode = "<?php\n".$proxyCode;
                 $proxyCode = substr(Kernel::stripComments($proxyCode), 5);