Views rows source: a single row loses its list structure in a slot

Problem/Motivation

With the Component views style, "Force using fields" checked and the "[View] Rows" source in a slot, a view with one result gives the slot the fields of that row instead of a list with one row. A template that loops on the slot then loops on the fields.

Steps to reproduce

1. Create a view with the Component style and check "Force using fields".
2. Pick a component whose template loops on a slot, and put the "[View] Rows" source in that slot.
3. Filter the view so it returns one result.

Proposed resolution

The component builder unwraps single-item arrays at every level, so it also removes the extra level the views source adds for a single row. We could unwrap one level only, then remove empty arrays as it does today.
Add a kernel test for a view with one row, and one for the comment form in a slot.