scrapers: drop Swiftly products with unparseable price

Products missing a parseable sale or regular price would previously yield
a GroceryItem with current_price=None. That broke the downstream matcher
(ingredient typical_price is non-null) and cluttered the table.

Added a guard in map_product() to return None when both reg_price and
sale_price are None. Fixes test_map_product_returns_none_for_unparseable.

backend/app/scraper/lucky_ca_scraper.py

@@ -259,6 +259,9 @@ class LuckyCaliforniaScraper:
         promo = price_block.get("promoArea") or {}
         sale_price, sale_unit = cls._parse_price(promo.get("promoText"))
 
+        if reg_price is None and sale_price is None:
+            return None
+
         unit = sale_unit or reg_unit
         is_on_sale = sale_price is not None and reg_price is not None and sale_price < reg_price
         # current_price = "what the customer pays today" → sale_price when on sale.