Fix 500 error accessing restricted visibility levels

Merged Stan Hu requested to merge sh-fix-error-500-explore into master

!22522 (merged) introduced a regression that caused a 500 error if the application_settings.restricted_visibility_levels column were nil. Since there is no NOT NULL constraint, there's no guarantee that it is populated with the proper serialized YAML data.

To fix this specific problem, we can account for nil values, but we should fix this at the database level.

Closes #215552 (closed)

Edited by Stan Hu