dillon1000/react

Clean up fastAddProperties and make it more correct (#29015)

## Summary

This PR makes some fixes to the `fastAddProperties` function:
- Use `if (!attributeConfig)` instead of `if (attributeConfig ===
undefined)` to account for `null`.
- If a prop has an Object `attributeConfig` with a `diff` function
defined on it, treat it as an atomic value to keep the semantics of
`diffProperties`.

## How did you test this change?

Build and run RNTester app.