Replace None in templates with placeholder filter

To be consistent, all uses of — or None is replaced with the
placeholder filter.

Fixes #9537

netbox/templates/dcim/devicerole.html

@@ -54,7 +54,7 @@
               {% if object.vm_role %}
                 <a href="{% url 'virtualization:virtualmachine_list' %}?role_id={{ object.pk }}">{{ virtualmachine_count }}</a>
               {% else %}
-                &mdash;
+                {{ ''|placeholder }}
               {% endif %}
             </td>
           </tr>