minor #21352 [DependencyInjection] Fix return of YamlFileLoader::parseDefaults (dunglas)

This PR was merged into the 3.3-dev branch.

Discussion
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[DependencyInjection] Fix return of YamlFileLoader::parseDefaults

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no (or very unlikely)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

I've messed up the return value of parseDefaults under certain conditions in #21342. Here is the fix... Sorry about that.

Commits
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5cf76f7 [DependencyInjection] Fix return of YamlFileLoader::parseDefaults

Author: nicolas-grekas

Loader/YamlFileLoader.php

@@ -175,15 +175,15 @@ private function parseDefinitions(array $content, $file)
     private function parseDefaults(array &$content, $file)
     {
         if (!isset($content['services']['_defaults'])) {
-            return $content;
+            return array();
         }
         if (!is_array($defaults = $content['services']['_defaults'])) {
             throw new InvalidArgumentException(sprintf('Service defaults must be an array, "%s" given in "%s".', gettype($defaults), $file));
         }
         if (isset($defaults['alias']) || isset($defaults['class']) || isset($defaults['factory'])) {
             @trigger_error('Giving a service the "_defaults" name is deprecated since Symfony 3.3 and will be forbidden in 4.0. Rename your service.', E_USER_DEPRECATED);
 
-            return $content;
+            return array();
         }
 
         $defaultKeys = array('public', 'tags', 'inherit_tags', 'autowire');