hide track if empty

this fixes #7

Author: fkusei

pretalx_halfnarp/templates/pretalx_halfnarp/frontend.html

@@ -46,7 +46,9 @@ <h2 class="title">{{submission.title}}</h2>
                     <p class="speakers">{{submission.display_speaker_names}}</p>
 
                     <div class="footer">
-                        <div class="track">Track: <span class="track">{{submission.track}}</span></div>
+                        {% if submission.track %}
+                            <div class="track">Track: <span class="track">{{submission.track}}</span></div>
+                        {% endif %}
                         <a href="{{submission.urls.review}}" target="_blank">{% translate "More info" %}</a>
                     </div>
                 </div>