do not coerce bonus or assessment into booleans

[davidsbailey]

See #38440 for context. allow nil values for bonus and assessment, so that we do not end up bouncing between nil and false for these values when the script and lesson edit pages are saved.

Testing story

Manually verified that saving the lesson edit page does not change these values back to false. Added test coverage which requires the code change in order to pass. The failing UI test in drone appears to be unrelated to this PR.

Reviewer Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked