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).

fixes #37111

Author: SimonHeimberg

DependencyInjection/FrameworkExtension.php

@@ -1213,7 +1213,9 @@ 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;