Closes #34924

Now, when `local_paths` are empty, we detach `session_id` from that
workspace serialization item. This way, when we restore it using the
default "last_session", we don't restore this workspace back. This is
same as when we use `cmd-w` to close window, which also sets
`session_id` to `None` before serialization.

Release Notes:

- Fixed an issue where last removed folder from workspace used to reopen
on Zed startup.