Reduce Sentry errors by adding defensive checks

[bjester]

Summary

Description of the change(s) you made

• Fixes regression in channel details API where it returns lists of falsy values (sentry)
• Enforces items prop is required for ExpandableList and fixes defaulting to empty lists in channel details endpoint (sentry)
• Prevents access to string method on null value when checking a task's channel ID (sentry)
• Checks assessment vuex map exists before calling delete of a key (sentry)
• Delays rendering of UI elements until fully loaded (sentry)

Manual verification steps performed

1. Add sleep(20) to ContentNodeViewSet.get_queryset
2. Clear your browsers cache or open incognito window
3. Sign into studio and open a channel
4. Observer not all interface components, like Publish, Add, or menus render until fully loaded

Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
• If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[rtibbles]

Code changes make sense, manual testing checks out.

[rtibbles]

Ah yeah - I fixed this on the frontend, but better to just not return this data at all.