phase3: util_web + shop_scroll (custom scroll) null-safe; global Key? param fix

lib/widgets/mobile/product_item.dart

@@ -19,7 +19,7 @@ class ProductItem extends StatefulWidget {
   final double imageWidth;
 
   const ProductItem(this.product, this.business, {
-    Key key,
+    Key? key,
     this.horizontal = false,
     this.imageWidth = 110,
   }) : super(key: key);