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/zed/src/zed.rs

@@ -2126,7 +2126,6 @@ mod tests {
                 pane.update(cx, |pane, cx| {
                     drop(editor);
                     pane.close_active_item(&Default::default(), window, cx)
-                        .unwrap()
                 })
             })
             .unwrap();