rewrite react examples

[ivanjeremic]

rewrite react example to more correct react code, components should return null to prevent rendering instead of an empty Fragment <></>. More info: https://legacy.reactjs.org/docs/conditional-rendering.html#preventing-component-from-rendering

[sweep-ai[bot]]

Apply Sweep Rules to your PR?

• [ ] Apply: All new business logic should have corresponding unit tests.
• [ ] Apply: Refactor large functions to be more modular.
• [ ] Apply: Add docstrings to all functions and file headers.

[matteobruni]

Stale, closing