Ambiguity on why the index property in ViewToken is marked optional

Description

On the ViewToken page in the docs it shows that the index property is optional. This is not explained in any further detail by the docs.

What is the problem?

It is unclear to me in what situation this property would be null. This object is used as a parameter for the onViewableItemsChanged callback in VirtualizedList so it is in turn also used in both FlatList and SectionList. This makes me think there wouldn't be a situation where index would be null.

How can we address it?

Update the description of the index property on ViewToken to clarify in what situation it could be null.

If anyone knows a resource like an issue in the react-native repo or a from post that discusses this or can explain it to me I can also make a PR to clarify the description on the property.

Why is it important?

It is currently ambiguous why the index property on ViewToken is marked as optional.

Who needs this?

Anyone using the onViewableItemsChanged prop and relies on the indexes of the callback.

Start with the ViewToken page in the React Native documentation and inspect the description of its optional index property. Review how ViewToken is used by the onViewableItemsChanged callback in VirtualizedList, FlatList, and SectionList, then confirm the situation that permits a null index. Update the property description to explain that situation and verify the rendered documentation.