Return the correct value from target server groups when no server groups

Author: CarolynRountree

core/src/main/python/wlsdeploy/tool/util/target_helper.py

@@ -99,7 +99,7 @@ def target_server_groups_to_servers(self, server_groups_to_target):
 
         self.logger.entering(server_groups_to_target, class_name=self.__class_name, method_name=_method_name)
         if len(server_groups_to_target) == 0:
-            return None, None
+            return list(), list()
 
         location = LocationContext()
         root_path = self.alias_helper.get_wlst_attributes_path(location)