[FIX] models: attribute pool on a model's class's may be falsy but not None

Modify the test to match `None` instead of any falsy value.  The latter
triggers a bug when installing a just-deinstalled module (point_of_sale).

odoo/models.py

@@ -2768,7 +2768,7 @@ class BaseModel(object):
         cls = type(self)
         cls._setup_done = False
         # a model's base structure depends on its mro (without registry classes)
-        cls._model_cache_key = tuple(c for c in cls.mro() if not getattr(c, 'pool', None))
+        cls._model_cache_key = tuple(c for c in cls.mro() if getattr(c, 'pool', None) is None)
 
     @api.model
     def _setup_base(self, partial):