[fix] only remove a service if it is setted to null

data/hooks/conf_regen/01-yunohost

@@ -65,7 +65,7 @@ with open('/etc/yunohost/services.yml') as f:
 updated = False
 for service, conf in new_services.items():
     # remove service with empty conf
-    if not conf:
+    if conf is None:
         if service in services:
             print("removing '{0}' from services".format(service))
             del services[service]

data/templates/yunohost/services.yml

@@ -50,7 +50,7 @@ yunohost-firewall:
 nslcd:
    status: service
    log: /var/log/syslog
-nsswitch: null
+nsswitch: {}
 bind9: null
 tahoe-lafs: null
 memcached: null