How to add_filter only when content is not empty?

Question

Based on your edits, you want one function to apply to two different filters. All you need to do is use the same function on both filters. I haven’t tested this, but it should work:

function content_image_markup($content) {
    if( !empty($content) ) {
        // example of changing the content
        $content=$content . ' append this to content';
    }
    return $content
}

add_filter('the_content', 'content_image_markup', 15);
add_filter('acf_the_content', 'content_image_markup', 15);