Show button if meta box has content, else hide

by

Check to see if it’s empty before you show the div.

<?php $true_title = get_post_meta( $post->ID, 'true_title', true ); ?>

<?php if( ! empty( $true_title ) ) : ?>

  <div class="discuss"><a href="https://wordpress.stackexchange.com/questions/166858/<?php echo $true_title;  ?>">Discuss...</a></div>

<?php endif; ?>