Include Disqus script only if shortname is present in _config.yml

_includes/_disqus_comments.html

@@ -1,3 +1,4 @@
+{% if site.disqus_shortname %}
 <script type="text/javascript">
     /* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */
     var disqus_shortname = '{{ site.disqus_shortname }}'; // required: replace example with your forum shortname
@@ -19,3 +20,4 @@
 </script>
 <noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript>
 <a href="http://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a>
+{% endif %}