Describe the problem

At time of writing, the declarative schema changer does not validate the NOT NULL nature of newly added columns. The change in #81679 captures most of what is necessary: push the logic into the index backfiller. The remaining case that needs to be covered is validating the NOT NULL nature of virtual computed columns, which is being handled separately for the legacy schema changer.

Jira issue: CRDB-16031

Epic CRDB-43310