maybe this is the right condition

index.html

@@ -158,7 +158,7 @@ <h2>PhD dissertations</h2>
           <h2>Other material</h2>
         </div>
 
-        {% if site.data.publications.draft_papers %}
+        {% if site.data.publications.draft_papers.length > 0 %}
         <div>
           <p>Draft papers:</p>
           <dl class="ref">