Fix `QuitLastUnnamed()` checks
Fix `QuitLastUnnamed()` checks

`getbufvar(buf, "&name")` was always empty, because `'name'` isn't a Vim
option.

Instead, check that the product of the following things is `1`:
- `tabpagenr('$')`: `1` if there's only one tab page
- `winnr('$')`: `1` if there's only one window
- `empty(bufname(''))`: `1` if the current buffer has no name

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