Fix false positive when passing null to socket_select

[Forceu]

Fixes #6318

According to PHP documentation the function is

socket_select(
    ?array &$read,
    ?array &$write,
    ?array &$except,
    ?int $seconds,
    int $microseconds = 0
): int|false

however Psalm did not allow null for $seconds

[weirdan]

Thanks!