Investigate how global preprocess map can be null

Problem/Motivation

#3564510: TypeError: Drupal\Core\Utility\ThemeRegistry::getGlobalPreprocess(): Return value must be of type array highlights that the global preprocess' cache can be null.

This problem is this is instantiated with an empty array.
Berdir suggested a theme_restry_alter hook could be the culprit but I could not replicate.

Let's investigate the root cause here.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet