Revert constraints for add_column_with_defaults

What does this MR do?

Revert constraints for add_column_with_defaults

There is a consistency issue with this: The outcome of the migration (and whether or not we see a NOT NULL or a CHECK constraint) depends on when the migration is being run and in which state the helper is at that time. This includes a risk of seeing installations that have different constraints, in particular differing from what we expect to see in db/structure.sql.

As such, this commit reverts the line change introduced in !30943 (diffs).

A follow-up is !31736 (merged) which deprecates the add_column_with_default helper anyways. For that, it's helpful to keep using NOT NULL constraints (because the outcome is the same if we just delegate to add_column).