Fix panic when immediately closing a window while opening paths (#3092)

Fixes this panic that I've been seeing in Slack:


[example](https://zed-industries.slack.com/archives/C04S6T1T7TQ/p1696530575535779)


```
thread 'main' panicked at 'assertion failed: opened_items.len() == project_paths_to_open.len()'
crates/workspace/src/workspace.rs:3628
<backtrace::capture::Backtrace>::create
<backtrace::capture::Backtrace>::new
Zed::init_panic_hook::{closure#0}
std::panicking::rust_panic_with_hook
std::panicking::begin_panic_handler::{{closure}}
std::sys_common::backtrace::__rust_end_short_backtrace
_rust_begin_unwind
core::panicking::panic_fmt
core::panicking::panic
<workspace::Workspace>::new_local::{closure#0}::{closure#0}
```

I believe it was caused by a window being closed immediately, while it
was still loading some paths. There was a mismatch in expectation
between the `workspace::open_items` function (which contains this
assertion), and the `Workspace::load_workspace` method. That later
method can return an empty vector if the workspace handle is dropped
while it is executing.

Release Notes:

- Fixed a crash when closing a Zed window immediately after opening it