Bugfix: errors from repeatable subgroups

Fixed a bug where updating subgroups which resulted in a smaller set threw an error when attempting to preprocess empty padded values, which are passed to give the preprocessing a chance to run (see also #164).

Also moved `test_saving_nested_groups` out of the unrelated Datasource_Post class and into the new Group test class.
Ah yeah, I should have made that comment here in the meta vs actually in the code comments.