`RemoveHiddenDataPreview` with hidden repeating group sets row values to `null` instead of deleting rows

### Description of the bug

When setting `RemoveHiddenDataPreview` to `true` and hiding a repeating group, it might be expected that the repeating group is deleted (i.e. that all rows disappear), but instead the rows are kept and all values (leaf nodes) in the repeating structure are set to `null`.

This caused problems when receiving data on the other end for at least one customer, as null values here are not valid according to the data model.

### Steps To Reproduce

1. Open `mat/lakselus-rapportering`
2. Fill out the form, say you have at least one 'behandling'
3. Add at least one row in the 'behandlinger' repeating group
4. Regret your decision and say don't have any 'behandlinger' (thereby hiding the repeating group)
5. Submit the form
6. Observe the null-ed out rows in the data model

### Additional Information

- https://altinn.slack.com/archives/C047F8FK2NQ/p1697008355300669

Reproduce the behavior in mat/lakselus-rapportering by enabling RemoveHiddenDataPreview, adding a behandlinger row, hiding the repeating group, and submitting the form. Trace the code that processes hidden repeating groups and verify the completed data model contains no rows rather than rows whose leaf values are null.

Written by the indexing model from the issue text.