Wordpress query showing post in empty post__in array?

This isn't directly fixing the issue with post__in but I don't see why this wouldn't work..

if(!empty($postids)){
    $posts = new WP_Query(array(
        'post__in' => $postids,
        'meta_key' =>'ratings_average',
        'orderby'=>'meta_value_num',
        'order' =>'DESC',
    ));
} else {
    //Do something else or nothing at all..
}

as you can see the WP_Query call will only happen if $postids has value/s in it. if it doesn't, then no call is made to WP_Query and the loop will just never happen, same as if your query returned 0 posts.

As noted, wp devs don't want to fix this. Having said that, you could pass a non-empty array of invalid IDs, like this:

if(empty($postids)) {
    $postids = ['issue#28099'];
}

$posts = new WP_Query(array(
    'post__in' => $postids,
    'meta_key' =>'ratings_average',
    'orderby'=>'meta_value_num',
    'order' =>'DESC',
));

Bad practice you say ? Yeah, I am not sure from whose side though ...

To keep the flow correct with the WP_Query. Use it like this:

    $postIdArray = array(
        1, 2, 3
    );

    $queryArgs = array(
        'post_type' =>  'any',
        'post_status' => 'published',
        'post__in' => ((!isset($postIdArray) || empty($postIdArray)) ? array(-1) : $postIdArray)
    );

This way you will still be able to code against the WP_Query object.

For example:

    $postIdArray = array(
        1, 2, 3
    );

    $queryArgs = array(
        'post_type' =>  'any',
        'post_status' => 'published',
        'post__in' => ((!isset($postIdArray) || empty($postIdArray)) ? array(-1) : $postIdArray)
    );

    $postQuery = new \WP_Query($queryArgs);
    $postCount = $postQuery->post_count;
    $totalCount = $postQuery->found_posts;