fix(tonconnect): exclude undefined stateInit from proof response

[mois-ilya]

Summary

• Fix stateInit: undefined appearing in TonConnect proof response in Firefox extension
• Conditionally add stateInit only when defined

Problem

Firefox preserves undefined values during postMessage serialization, causing TonConnect SDK to see stateInit: undefined instead of the field being absent.

Solution

Use spread operator to conditionally include stateInit only when it's defined.

Fixes PRO-404

[github-actions]

Successful WEB deployment 🚀🚀🚀

Well done!
Link to test environment:
https://05c31753.tonkeeper-web.pages.dev

[Anonym-tsk]

lgtm