Removed AUTHOR defaulting to OS User /John Doe, so both a blank or undefined (None) author is possible.

Reverted templates back to checking author object, since a None object is possible. Name could be checked for blank if required
ATOM spec states an author element should be provided, so passes a blank name if not specified
Updated unit test

pelican/themes/simple/templates/article.html

@@ -10,7 +10,7 @@
     <abbr class="published" title="{{ article.date.isoformat() }}">
       {{ article.locale_date }}
     </abbr>
-    {% if article.author.name %}
+    {% if article.author %}
     <address class="vcard author">
       By <a class="url fn" href="{{ SITEURL }}/{{ article.author.url }}">{{ article.author }}</a>
     </address>