fix reference disappearing for distill blog in absence of author (alshedivat#1804)

Author: YifanJiang233

_layouts/distill.html

@@ -66,8 +66,9 @@
         <h1>{{ page.title }}</h1>
         <p>{{ page.description }}</p>
       </d-title>
-
+      {%- if page.authors -%}
       <d-byline></d-byline>
+      {%- endif -%}
 
       <d-article>
         {% if page.toc -%}