How to add_filter only when content is not empty?

Question

Im trying to add filter but only when the_content is not empty. So I was trying do something like that:

<?php
   $thecontent = get_the_content();
   if(!empty($thecontent)) {

   add_filter('the_content', 'gs_add_img_lazy_markup', 15);

} ?>

but without results. How should I do it propertly?

edit

What I want to do is create one function with two filters, but one this function should be done only when the_content is not empty. Right now I did this:

add_filter('the_content', 'content_image_markup', 15);
function content_image_markup($the_content) { 
$thecontent = get_the_content();
   if(!empty($thecontent)) {
        // function content
    }
}

add_filter('acf_the_content', 'acf_content_image_markup', 15); 
function acf_content_image_markup($the_content) {
        // the same function content what above         
}

how to makes this easer?

Answer 1

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);