If a site with a non-empty question bank is upgraded to 4.0, then rolled back by restoring a MySQL/MariaDB dump file, and the upgrade attempted again you get the error:

Default exception handler: Unknown DDL library error Debug: Field question_bank_entries->questionid cannot be added. Not null fields added to non empty tables require default value. Create skipped
Error code: ddlunknownerror
* line 540 of /lib/ddl/database_manager.php: ddl_exception thrown
* line 3935 of /lib/db/upgrade.php: call to database_manager->add_field()
* line 1875 of /lib/upgradelib.php: call to xmldb_main_upgrade()
* line 196 of /admin/cli/upgrade.php: call to upgrade_core()

There are two issues here:

1. I think the upgrade should fail because mdl_question_bank_entries already exists but that's not what's causing the above error.
2. The code has a mistake which results in this error (column questionid incorrectly created as NOT NULL because a bogus constant is passed to $notnull) so actually this error shouldn't happen at all.

Steps to reproduce this

1. Set up a Moodle 3.11 site using MySQL or MariaDB for the database.
2. Add a course.
3. Add a quiz to the course.
4. Add a question to the quiz.
5. Back up the database using mysqldump (for example mysqldump -u root -p --single-transaction --default-character-set=utf8mb4 moodle > moodle.sql).
6. Upgrade the site to Moodle 4.0.
7. Restore the database from step 5 using the mysql command (for example mysql -u root -p moodle < moodle.sql).
8. Upgrade the site again to Moodle 4.0.

Expected results

I would expect the error Default exception handler: Table "question_bank_entries" already exists.

Actual results

Error Default exception handler: Unknown DDL library error Debug: Field question_bank_entries->questionid cannot be added. Not null fields added to non empty tables require default value. Create skipped.