CMFActivity.Activity.SQLBase: Expect group_method_id to always be a string.

group_method_id column is declared "NOT NULL", so it can only be None if
getProcessableMessageList could not find any activity, in which case this
code is not executed ("if message_list" is false).
This simplifies two "if" statements.

product/CMFActivity/Activity/SQLBase.py

@@ -624,9 +624,8 @@ CREATE TABLE %s (
         node_family_id_list)
     if message_list:
       # Remove group_id parameter from group_method_id
-      if group_method_id is not None:
       group_method_id = group_method_id.split('\0')[0]
-      if group_method_id not in (None, ""):
+      if group_method_id != "":
         method = activity_tool.invokeGroup
         args = (group_method_id, message_list, self.__class__.__name__,
                 hasattr(self, 'generateMessageUID'))