[6.x] Fix bug caused by localisation refactoring (#29967)

* Fix bug caused by localisation refactoring

In Laravel 5.8 calling __(null) would return null. Due to the code refactoring __(null) now returns a Translator object. As this is not mentioned in the Upgrade Guide I presume this is an unintended bug. This pull request resolves the issue.

Details:
In Laravel 5.8 you could run the following code in a blade:
    @Dump(trans(null))  // Returns Translator object in 5.8 and 6.
    @Dump(__(null))  // Returns null in 5.8 and Translator object in 6.

    {{ __(null) }} // Runs successfully on 5.8 fails on 6.0 see error below.

Facade\Ignition\Exceptions\ViewException
htmlspecialchars() expects parameter 1 to be string, object given (View: resources\views\welcome.blade.php)

If it is intentional the upgrade guide should be updated to put an if statement checking for null for each translatable variable that could contain null.

* Fixed code style

src/Illuminate/Foundation/helpers.php

@@ -906,6 +906,10 @@ function trans_choice($key, $number, array $replace = [], $locale = null)
      */
     function __($key = null, $replace = [], $locale = null)
     {
+        if (is_null($key)) {
+            return $key;
+        }
+
         return trans($key, $replace, $locale);
     }
 }