Top border visible in split lists

**Describe the bug**

We recently removed top border from lists, this causes issues in split variant of list that displays in 2 columns:

Top element on second column has top border, because it's not the first element in the list.
I don't know if there is a way in CSS to know if element is first of given column to fix that…

Start by locating the split variant of the list styles and reproduce the two-column rendering shown in the issue. The work is done when the top item in each column has no unintended top border while the list's other borders remain correct; no test file or entry point is named.

Written by the indexing model from the issue text.