'undefined' returned instead of undefined in string split with regex

[malbarbo]

node and deno evaluates the expression "a ".split(/(d)*/) to [ "a", undefined, " " ], but quickjs-ng evaluates to [ 'a', 'undefined', ' ' ].

[bnoordhuis]

Kind of surprising because e.g. exec() does work correctly - and apparently test262 doesn't test it? Doubly surprising.

Anyway, legit bug - I'll fix it.