Ensure item-closing actions do not panic when no items are present (#31845)

This PR adds a comprehensive test that ensures that no item-closing
action will panic when no items are present. A test already existed
(`test_remove_active_empty `) that ensured `CloseActiveItem` didn't
panic, but the new test covers:

- `CloseActiveItem`
- `CloseInactiveItems`
- `CloseAllItems`
- `CloseCleanItems`
- `CloseItemsToTheRight`
- `CloseItemsToTheLeft`

I plan to do a bit more clean up in `pane.rs` and this feels like a good
thing to add before that.

Release Notes:

- N/A

crates/file_finder/src/file_finder_tests.rs

@@ -739,7 +739,6 @@ async fn test_ignored_root(cx: &mut TestAppContext) {
         .update_in(cx, |workspace, window, cx| {
             workspace.active_pane().update(cx, |pane, cx| {
                 pane.close_active_item(&CloseActiveItem::default(), window, cx)
-                    .unwrap()
             })
         })
         .await