Check if the image size is available and if not use ‘full’ image size

I don’t see why something like this wouldnt work!

$med_post_thumb = get_the_post_thumbnail($post->ID, 'medium_post_thumbnails', array('class' => 'img-fluid')). '</a></div>';
$full_post_thumb = get_the_post_thumbnail($post->ID, 'FULL SIZE GOES HERE', array('class' => 'img-fluid')). '</a></div>';

if ($med_post_thumb == NULL) 
{
     echo $full_post_thumb;
} 
else
{
     echo $med_post_thumb;
}