Drupal - Get site name config value in page.html.twig template

I am only able to get null values when I try to get the value in the preprocess function.

Null values based on doing what exactly?

The site name, logo etc. was moved to a block, that's why it's no longer available out of the box. However, you can always add it back yourself and make it available in twig:

$variables['site_name'] = \Drupal::config('system.site')->get('name');

It looks like you can also use the following technique with twig_tweak

{{ drupal_config('system.site', 'name') }}