bug #40744 allow null for framework.translator.default_path (SimonHeimberg)

This PR was merged into the 4.4 branch.

Discussion
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allow null for framework.translator.default_path

Allow to configure framework.translator.default_path to null, as it was until symfony 3.4.x (fix BC compatibility).

| Q             | A
| ------------- | ---
| Branch?       | 4.4 for bug fixes <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #37111 <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT
| Doc PR        | no <!-- required for new features -->

Commits
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6fd21ce5aa allow null for framework.translator.default_path

Author: fabpot

DependencyInjection/FrameworkExtension.php

@@ -1216,7 +1216,9 @@ private function registerTranslatorConfiguration(array $config, ContainerBuilder
             $container->getDefinition('console.command.translation_update')->replaceArgument(6, $transPaths);
         }
 
-        if ($container->fileExists($defaultDir)) {
+        if (null === $defaultDir) {
+            // allow null
+        } elseif ($container->fileExists($defaultDir)) {
             $dirs[] = $defaultDir;
         } else {
             $nonExistingDirs[] = $defaultDir;