[IMP] base_kanban_stage: Centralize default stage ID

* Centralize default_stage_id and return empty RecordSet of type `base_kanban_stage`

base_kanban_stage/models/base_kanban_abstract.py

@@ -99,7 +99,7 @@ class BaseKanbanAbstract(models.AbstractModel):
 
     @api.model
     def _default_stage_id(self):
-        return  # pragma: no cover
+        return self.env['base.kanban.stage']
 
     @api.multi
     def _read_group_stage_ids(

base_kanban_stage/tests/test_base_kanban_abstract.py

@@ -64,3 +64,10 @@ class TestBaseKanbanAbstract(TransactionCase):
                 {self.id_1: False, self.id_2: True},
             )
         )
+
+    def test_default_stage_id(self):
+        ''' It should return an empty RecordSet '''
+        self.assertEqual(
+            self.env['base.kanban.abstract']._default_stage_id(),
+            self.env['base.kanban.stage']
+        )