bug #24405 [Intl] fix return type value (xabbuh)

This PR was merged into the 4.0-dev branch.

Discussion
----------

[Intl] fix return type value

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

Commits
-------

0e1ce34102 [Intl] fix return type value

src/Symfony/Component/Intl/Locale.php

@@ -105,6 +105,8 @@ final class Locale extends \Locale
         if (strlen($locale) < 4) {
             return self::$defaultFallback;
         }
+
+        return null;
     }
 
     /**