Fix filter return provider

[klimick]

Currently $result infers to array<int, int>:

/** @var $numbers list<int|null> */
$numbers = [1, 2, null];
$result = filter($numbers, fn(int|null $n) => null !== $n)

But return type should be list<int> (because preserveKeys false by default)

Furthermore. In this case, $result infers to array<string|int, int>:

/** @var $numbers list<int|null> */
$numbers = [1, 2, null];
$result = filter($numbers, fn(int|null $n) => null !== $n, preserveKeys: (bool) rand(0, 1))

But should be array<int, int> (not list<int> because we cannot be sure about preserveKeys value).

PR fixes errors from above.