Show only child categories?

Question

WordPress

I found on the net such a code for displaying only child categories. Those. if the entry belongs to the category: Parent rubric - Child rubric, then only the Child rubric will be displayed. For example, Books - Detectives, then this code will display only a link to the child category Detectives.

<?php
    $parentscategory ="";
    foreach((get_the_category()) as $category) {
    if ($category->category_parent > 0) {
    $parentscategory .= '<a href="' . get_category_link($category->cat_ID) . '">' . $category->name . '</a>, ';
    }
    }
    echo substr($parentscategory,0,-2);
?>

The problem is that if the entry belongs to a category that does not have a child category, then nothing is displayed.
Please tell me how to modify this code so that if a post does not have a child category, then its Parent category is displayed, and not an empty space, as it is now. Thank you!