User profile page: only display providers if there are some

Someone uses uMap with classic auth username/password, and they
don't want those empty HTML elements

umap/templates/auth/user_form.html

@@ -19,6 +19,7 @@
         <input type="submit" value="{% trans 'Save' %}" />
       </form>
     </div>
+    {% if backends.backends|length %}
       <div class="row">
         <h3>{% trans "Your current providers" %}</h3>
         <ul>
@@ -44,5 +45,6 @@
           </ul>
         </div>
       </div>
+    {% endif %}
   </div>
 {% endblock maincontent %}