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/editor/src/editor_tests.rs

@@ -20580,7 +20580,6 @@ async fn test_invisible_worktree_servers(cx: &mut TestAppContext) {
     pane.update_in(cx, |pane, window, cx| {
         pane.close_active_item(&CloseActiveItem::default(), window, cx)
     })
-    .unwrap()
     .await
     .unwrap();
     pane.update_in(cx, |pane, window, cx| {