Fallback to showing first published date if absence of other info

First publication date is only shown when the editor is not known
(rationale: if the editor for the first edition is known, the same
date should be set in both fields).

bookwyrm/templates/book/publisher_info.html

@@ -66,6 +66,10 @@
         {% blocktrans %}Published {{ date }}{% endblocktrans %}
     {% elif publisher %}
         {% blocktrans %}Published by {{ publisher }}.{% endblocktrans %}
+    {% elif book.first_published_date %}
+        {% with date=book.first_published_date|naturalday %}
+        {% blocktrans %}Published {{ date }}{% endblocktrans %}
+        {% endwith %}
     {% endif %}
 </p>
 {% endif %}