Revert "return list for _get_optional_services() (sonic-net#822)" (sonic-net#848)

This reverts commit f1c79d5.

Author: lguohan

config/main.py

@@ -413,6 +413,8 @@ def _get_optional_services():
     config_db = ConfigDBConnector()
     config_db.connect()
     optional_services_dict = config_db.get_table('FEATURE')
+    if not optional_services_dict:
+        return None
     return optional_services_dict.keys()
 
 def _stop_services():