Upgrade from book 2.0.x fails with TypeError type string on string in book_post_update_allowed_child_type_default6 LIVRE

Clone of #3563266: Upgrade from book 2.0.x fails with TypeError type string on string in book_post_update_allowed_child_type_default6

Same issue inherited from book. we'll fix it in livre first.

Problem/Motivation

Upgrading from a fresh book 2.0.x installation.

Getting this error:
TypeError: Cannot access offset of type string on string in book_post_update_allowed_child_type_default6

Stack of errors:

 [error] TypeError: Cannot access offset of type string on string in book_post_update_allowed_child_type_default6() (line 148 of /www/web/modules/contrib/book/book.post_update.php) #0 /www/vendor/drush/drush/src/Commands/core/UpdateDBCommands.php(291): book_post_update_allowed_child_type_default6(Array)
> #1 /www/vendor/drush/drush/includes/batch.inc(257): Drush\Commands\core\UpdateDBCommands::updateDoOnePostUpdate('book_post_updat...', Array)
> #2 /www/vendor/drush/drush/includes/batch.inc(204): _drush_batch_worker()
> #3 /www/vendor/drush/drush/includes/batch.inc(75): _drush_batch_command('1')
> #4 /www/vendor/drush/drush/src/Commands/core/UpdateDBCommands.php(145): drush_batch_command('1')
> #5 [internal function]: Drush\Commands\core\UpdateDBCommands->process('1', Array)
> #18 {main}.
> TypeError: Cannot access offset of type string on string in /www/web/modules/contrib/book/book.post_update.php on line 148 #0 /www/vendor/drush/drush/src/Commands/core/UpdateDBCommands.php(291): book_post_update_allowed_child_type_default6(Array)
> #1 /www/vendor/drush/drush/includes/batch.inc(257): Drush\Commands\core\UpdateDBCommands::updateDoOnePostUpdate('book_post_updat...', Array)
> [warning] Drush command terminated abnormally.

In ProcessBase.php line 171:

Unable to decode output into JSON: Syntax error

TypeError: Cannot access offset of type string on string in book_post_update_allowed_child_type_default6() (line 148 of /www/html/modules/contrib/book/book.post_update.php).

Steps to reproduce

Install book 2.0.x with this configuration:
book.settings.yml

allowed_type_book: 1
block:
  navigation:
    mode: 'all pages'
child_type_book: book

Then upgrade to book 3.0.x

Then observe bug.

Proposed resolution

See code in merge request

Remaining tasks

Review code in merge request

User interface changes

Allows upgrades without crashing

API changes

See merge request

Data model changes

See merge request