Use 'never' type for empty array literals

[ahejlsberg]

This PR introduces a widening form of the never type that we use for empty array literals in --strictNullChecks mode. This fixes the following issue reported in #8878:

// Compile with --strictNullChecks
function concat<T>(xs: T[], ys: T[]): T[] {
    return [...xs, ...ys];
}
const y = concat([], ["a"]);  // Should be type string[]

Before this fix the inferred type above was (string | undefined)[] because we were using type undefined[] for empty array literals, and undefined isn't a bottom type in strict null checking mode.

[mhegazy]

👍

[DanielRosenwasser]

Leave a comment on what a widening never implies over an unwidened never and why you need it.

[ahejlsberg]

Superceded by #8944.