fabric: don’t set bond slaves if there are none

Not that that should happen except by mistake.

roles/fabric/templates/switch.intf.j2

@@ -12,7 +12,9 @@ iface {{ iface.name }}
 {% if iface.type.value == 'lag' %}
 {#- Bond/MLAG stuff. #}
 {% set members = interfaces | selectattr('lag') | selectattr('lag.name', '==', iface.name) %}
+{% if members %}
     bond-slaves {{ members | map(attribute='name') | join(' ') }}
+{% endif %}
 {% if iface.custom_fields.bond_mode %}
     bond-mode {{ iface.custom_fields.bond_mode }}
 {% endif %}