Prevent creating empty objects

[erinknight242]

When you delete all child rows from an object from the modal window, it appropriately disables the "Update Model" button to prevent you from saving an object with no children.

However, if you edit a nested field (or model) from the root grid and delete it, it leaves an empty object if it was the only child of its parent. This is probably OK, because it would be frustrating to not be able to delete a row that you're trying to delete just because of how you got to the row. If we don't want to disable deleting only children for that reason, then we need to add some validation that would prevent you from saving a contract with an empty object.