## 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.
b37e4b4e61