when I pass _\flatten an array which contains empty array, _\flatten does not expand the empty array.

ex.

use function _\flatten;

$arr = [
    [1, 2, 3],
    [],
    [4, 5, 6],
];
$result = flatten($arr);

I expected that $result is [1, 2, 3, 4, 5, 6], but actually [1, 2, 3, [], 4, 5, 6].

this behavior is expected?