fix(react): Fix empty conditional slots hydration issues

[dmgawel]

Changes

• Fix React slot rendering to skip empty SlotString values and avoid emitting empty <astro-slot> nodes (prevents React hydration mismatches for conditional slots).
• Add a regression fixture (slots.astro + ConditionalSlot.jsx) and test to validate empty slots don’t render <astro-slot> while filled slots still do.

Minimal example that was causing hydration error:

  <ReactComponent client:load>
    {show ? <span slot="my-slot-name">Visible</span> : null}
  </ReactComponent>

Testing

New test case added.

Docs

• No docs updates needed (internal integration fix + tests only).

[changeset-bot]

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Latest commit: fb2dcae

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[matthewp]

@dmgawel this looks great! Can you add a changeset? pnpm changeset

[dmgawel]

@matthewp Done ✅