(Late) Follow-up to
https://github.com/zed-industries/zed/pull/26893#discussion_r2073427393

The mentioned issue of needed zero-division for scrollbars is now fixed
via #30189.
However, whilst the linked PR fixed the issue for the layouting of the
scrollbar thumb, I sadly did not address the (somewhat rare) case of
`document_lines == visible_editor_lines` within the calculation of the
minimap top offset.

This PR adds coverage for that case and ensures that the
`minimap_top_offset` never ends up being `NaN`.

Release Notes:

- N/A