feat(data-warehouse): Set nullable status on fields

[Gilbert09]

Problem

• Nullable fields get defaulted to being non-nullable, and thus get a default value
• In this support ticket, a users timestamp field gets defaulted to 1970-01-01

Changes

• Actually take whether the field is nullable into account

Does this work well for both Cloud and self-hosted?

Yes

How did you test this code?

Unit tests