really properly control return value of action_validate

inventory/models.py

@@ -417,10 +417,10 @@ class CagetteInventory(models.Model):
 
                 # Set inventory as 'done' even if some products missed
                 res = api.execute('stock.inventory', 'action_validate', [inv])
-                if res is None:
+                if not res:
                     done.append('Closed inventory')
                 else:
-                    action_validate_issue = "Action validate did not return None."
+                    action_validate_issue = "Action validate returned something."
                     coop_logger.error(action_validate_issue)
                     errors.append(action_validate_issue)
                     if isinstance(res, dict) and 'name' in res: