fix(client): make balance_callback call optional

[carlosmiei]

• relates to Handle optional balance_callback since we have all_updates_callback #951

[Copilot]

Pull Request Overview

This PR fixes a potential runtime error by making the balance_callback call optional when handling WebSocket user data events. The change adds a null check before invoking the balance_callback to prevent errors when the callback is not defined.

• Adds conditional check for balance_callback existence before invocation
• Aligns balance_callback handling with other optional callbacks in the same function

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}