[FIX] base_kanban_stage: Add pragma no cover

* Add pragma no cover to `None` return in method

base_kanban_stage/models/base_kanban_abstract.py

@@ -99,7 +99,7 @@ class BaseKanbanAbstract(models.AbstractModel):
 
     @api.model
     def _default_stage_id(self):
-        return
+        return  # pragma: no cover
 
     @api.multi
     def _read_group_stage_ids(