Fix fastAddProperties to properly nullify style props (facebook#30334)

## Summary

This PR fixes the `fastAddProperties` function. Now it nullifies a prop
if it was defined in one of the items of a style array, but then set to
`undefined` or `null` in one of the subsequent items. E.g. `style:
[{top: 0}, {top: undefined}]` should evaluate to `{top: null}`. Also
added a test case for that.

## How did you test this change?
```
yarn test packages/react-native-renderer -r=xplat --variant=false
yarn test packages/react-native-renderer -r=xplat --variant=true
yarn flow native
```

Author: felixshiftellecon