Improve soft-delete filtering on m2os

[stephenh]

Currently our m2os do not filter out soft-deleted entities with the rationale that, for required columns, it would change the return type from Book to Book | undefined.

However, for optional FKs, like ProjectItem.task in our domain, it's more okay to return undefined, b/c the type is already M2O<Task | undefined>.

Although maybe we should also consider it being invalid for a non-SD entity to point to a SD entity in the first place, and require the PI.task reference to be cleared in the db.

See Internal slack thread