Avoid displaying "None" in member detail view

In case the member has never paid a fee.

coin/members/templates/members/detail.html

@@ -54,7 +54,13 @@
                     <span class="label alert">non à jour !</span>
                 {% endif %}
             </p>
-            <p>Date de fin de cotisation : {{ user.end_date_of_membership }}</p>
+            <p>
+                {% if  user.end_date_of_membership %}
+                Date de fin de cotisation : {{ user.end_date_of_membership }}
+                {% else %}
+                Je n'ai encore jamais cotisé.
+                {% endif %}
+            </p>
 
             <a href="{{ membership_info_url }}" target="_blank" class="button small radius expand"><i class="fa fa-heart"></i>
  Renouveler ma cotisation</a>