Fix issue with setting initialSelectedindex to 0 in Flatlist

[Saadnajmi]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

There was a bug if you set initialSelectedIndex to 0 on Flatlist. Turns out this was the issue:

0 || -1; // returns -1
0 ?? -1; // returns 0

Derp

Changelog

[macOS] [Fixed] - Fixed issue with setting initialSelectedIndex to 0 on Flatlist

Test Plan

Tested the cases undefined, 0, 1, 2

Screen.Recording.2022-08-30.at.4.16.57.PM.mov

[amgleitman]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator

Yep, JavaScript is silly.