[6.x] Fix bug caused by localisation refactoring

[marijnkampf]

In Laravel 5.8 calling __(null) would return null. This is no longer the case in Laravel 6.

Calling __(null) now returns a Translator object due to the Translator code refactoring; $this->getFromJson(...) is no longer calling $this->makeReplacements(...).

As this is not mentioned in the Upgrade Guide and the documentation at https://laravel.com/docs/5.7/localization#retrieving-translation-strings states:

If the specified translation string does not exist, the __ function will return the translation string key. So, using the example above, the __ function would return messages.welcome if the translation string does not exist.

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 as it expects a string not an object, see full 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 any variables that are translated that could contain null.