fix: empty purchasedIds causing NOT IN () SQL syntax error

cfc-backend/src/main/java/com/etotem/cfc/service/ProductRecommendationService.java

@@ -110,7 +110,7 @@ public class ProductRecommendationService {
                             .eq(Product::getDomain, dimensionCode)
                             .eq(Product::getStatus, "上架")
                             .gt(Product::getStock, 0)
-                            .notIn(purchasedIds.isEmpty(), Product::getId, purchasedIds)
+                            .notIn(!purchasedIds.isEmpty(), Product::getId, purchasedIds)
             );
             candidates.addAll(domainMatched);
         }