Fixes testing if authors are present

bookwyrm/templates/book/book.html

@@ -43,7 +43,7 @@
                 </p>
             {% endif %}
 
-            {% if book.authors %}
+            {% if book.authors.exists %}
                 <div class="subtitle">
                     {% trans "by" %} {% include 'snippets/authors.html' with book=book %}
                 </div>

bookwyrm/templates/snippets/book_titleby.html

@@ -2,7 +2,7 @@
 {% load utilities %}
 {% spaceless %}
 
-{% if book.authors %}
+{% if book.authors.exists %}
 {% blocktrans trimmed with path=book.local_path title=book|book_title %}
 <a href="{{ path }}">{{ title }}</a> by
 {% endblocktrans %}