config_pages_config() can return NULL

Problem/Motivation

config_pages_config() can return NULL but the docblock does not acknowledge that.

Steps to reproduce

When doing something like this:

if ($page = config_pages_config('foo')) {
  // Do something with it
}

drupal-check is complaining:

If condition is always true.

Proposed resolution

The return type should be \Drupal\config_pages\Entity\ConfigPages|null.

Remaining tasks

I will submit a patch.