Add conditional check for citation URLs in citations.html

Author: hossam26644

citations.html

@@ -17,7 +17,9 @@ <h2 class="section__heading">How to cite tskit-dev software</h2>
       </div>
     </div>
     {% for app in page_apps %}
+      {% if app.citation_url %}
         {% include software-citation.html app=app %}
+      {% endif %}
     {% endfor %}
   </div>
 </section>