fix: make sure initialScrollIndex is bigger than 0 (#36844)

Summary:
Hey,

`adjustCellsAroundViewport` function was checking if `props.initialScrollIndex` is truthy and -1 was returning true. This caused bugs with rendering for tvOS: https://github.com/react-native-tvos/react-native-tvos/pull/485 There are warnings in the code about `initalScrollIndex` being smaller than 0 but this if statement would still allow that.

## Changelog:

[General] [Fixed] - Make sure initialScrollToIndex is bigger than 0 when adjusting cells

Pull Request resolved: https://github.com/facebook/react-native/pull/36844

Test Plan: Pass -1 as initialScrollToIndex. Check that this code is executed.

Reviewed By: cipolleschi

Differential Revision: D44856266

Pulled By: NickGerleman

fbshipit-source-id: 781a1c0efeae93f00766eede4a42559dcd066d7d