Enables restoration of empty workspaces (without folders) that contain
unsaved items like drafts or single files. Empty workspaces are now
identified by workspace_id rather than paths, allowing multiple empty
workspaces to coexist and be properly restored on startup. This ensures
users don't lose work when closing Zed with unsaved files in empty
workspaces.

Closes #15098

Release Notes:

- Improved: Empty workspaces with unsaved files now restore on startup
(hot-exit)

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Co-authored-by: Kirill Bulatov <kirill@zed.dev>