fix for empty bridge

files/interfaces.j2

@@ -58,7 +58,7 @@ iface {{vlan.link}}.{{vlan.tag}} inet6 manual
 auto {{br.name}}
 allow-hotplug {{br.name}}
 iface {{br.name}} inet {{br.type}}
-{% if br.links %}
+{% if br.links and br.links | length > 0 %}
 bridge_ports{% for link in br.links %} {{link}}{% endfor %}
 
 {% endif %}