Add List.getOrNull()

[Yuxiang Liu]

add a extension function getOrNull(int index) on List to get element at the specified index safely. If out of bounds, return null.
It can work well with nnbd.

ref: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/get-or-null.html

[alexeyinkin]

This should be on Iterable since elementAt is there. Also it might be named elementAtOrNull for consistency.