fix: Promotion discounts with uninitialized filter keys no longer crash the cart

fix: Promotion discounts with uninitialized filter keys no longer crash the cart: a task in bird-critic-1.5-full-300 (Harbor dataset). PromotionDiscountEntity has typed string properties for its sorter and applier keys. When a promotion discount is saved through the administration UI with…

The task

`PromotionDiscountEntity` has typed string properties for its sorter and applier keys. When a promotion discount is saved through the administration UI with `considerAdvancedRules` enabled but without selecting values for the "Apply to" and "Sort by" dropdowns, those properties are stored as `NULL` in the database.…

Part of kochsnow/bird-critic-1.5-full-300.