Fix variable errors when including settings file the second time

Problem/Motivation

Report by @Dave Reid.

So the specific error I'm seeing in our settings.php is that when we attempt to include it the second time, the variable $config is NULL, instead of an array. It looks like setting the same three variables to empty arrays, just like \Drupal\Core\Site\Settings::initialize() does resolves it locally for me. And gives a cleaner inspection of the actual settings as well.

This could also be fixed by using include_once or require_once in our settings.php file of our sub-settings files, which we will also do in our code, but the attached patch I would suggest for inclusion and release.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes