Display excerpt if subheading field isn't filled out

I’m pretty sure this is a basic thing… but I can’t for the life of me figure it out.

By default just the title and subtitle are displayed. I want to set it up so that in the event that a content writer forgets to put in a subhead, it automatically pulls in the excerpt.

This is what I’ve got so far

<?php   if($post_format == 'video' || $post_format == 'audio')
        echo codeless_text_limit(get_the_content(), 60);
        elseif(
          function_exists('the_subheading')) { the_subheading(); }
        else{
           echo get_the_excerpt();} ?>

https://www.ads-software.com/plugins/subheading/