dock: Keep an empty StackPanel a stack when dumped (#2686)

## Description

`StackPanel::dump` assigned `state.info` **inside** the loop over its
children, so a StackPanel holding no panel never ran it and kept
`PanelState`'s default, `PanelInfo::Panel(Null)`:

```json
{ "panel_name": "StackPanel", "children": [], "info": { "panel": null } }
```

Restoring that takes the `PanelInfo::Panel` arm, which looks
`StackPanel` up in `PanelRegistry` — a container is never registered
there — so the whole tab renders *"The `StackPanel` panel type is not
registered in PanelRegistry."* instead of the layout.

Only reachable once a layout can be empty, which is why it went
unnoticed: a split layout built from a template always ships with
panels, and an empty canvas centre is `Tiles`, not `StackPanel`.
Downstream it shows up as a saved-but-empty split layout coming back
broken after a restart.

Assigning the info once before the loop also stops it being recomputed
on every iteration.

## Break Changes

None.

## How to Test

```
cargo test -p gpui-component dock::tab_panel   # 22 passed
cargo check --workspace --all-targets
cargo fmt --check
```

`empty_center_round_trips_as_a_stack` is new: it dumps a fresh
`DockArea` and asserts the centre comes back as `PanelInfo::Stack`. It
fails on `main`.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [ ] Passed `cargo run` for story tests related to the changes — no
story starts from an empty split layout.
- [ ] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)

AI assistance: written with Claude Code, reviewed by hand.