objects: fix ip address retrieval

Make sure that if the object is invalide, the
function returns an empty IP list.

This fixes an issue with creation of port forward rules
not using objects.

src/nethsec/objects/__init__.py

@@ -113,6 +113,8 @@ def get_object_ips(uci, database_id):
         a list of unique IP addresses from the object
     """
     ips = []
+    if not database_id:
+        return ips
 
     obj = get_object(uci, database_id)
     database, id = database_id.split('/')