content_moderation_post_update_update_cms_default_revisions fails if content_moderation was enabled but no entity types were being moderated

Problem/Motivation

To reproduce this issue:

git checkout 8.4.0 && composer install
drush si standard -y
drush en content_moderation -y
git checkout 8.6.x && composer install
drush updb

[notice] Update started: content_moderation_post_update_update_cms_default_revisions
[warning] array_shift() expects parameter 1 to be array, null given content_moderation.post_update.php:37
[ok] Update completed: content_moderation_post_update_update_cms_default_revisions

This is annoying because it prevents us from being able to use drupal-8.4.0.bare.standard.php.gz and drupal-8.4.0-content_moderation_installed.php together without an error being triggered. This is a blocker for #2949964: Add an EntityOwnerTrait to standardize the base field needed by EntityOwnerInterface.

Proposed resolution

Fix the post_update hook to work when no entity types are being moderated.

Remaining tasks

User interface changes

API changes

Data model changes