erp5_production_planning: return empty is not defined

bt5/erp5_production_planning/SkinTemplateItem/portal_skins/erp5_production_planning/ProductionPlanning_getCellRange.py

@@ -19,6 +19,8 @@ date_dict = {
  'saturday': 6,
  'sunday': 0
 }
+if not start_date:
+  return [],[]
 tmp_date = start_date
 day_number = tmp_date.dow()
 if day_number < date_dict[period]: