DARK: fix the id field is required (#423)

## QA Steps
[Thread
link](https://youcan-shop.slack.com/archives/C04BDG9HEB1/p1704305203081529)

- [ ] go to the product page and try to buy or add a product to the
cart.
-  [ ] verifie that the product will be added to the cart

## Note

Fix the issue on product page when u try to buy a product

sections/product.liquid

@@ -455,7 +455,7 @@
      * Well technically, if the seller didn't choos a default variant, we shouldn't select one.
      * The customer should be prompted to select one, if not, they get a warning message.
      */
-    const defaultVariant = variants.find(variant => variant.is_default);
+    const defaultVariant = variants.find(variant => variant.is_default) || variants[0];
     const globalProductStr = `{{ selected_product | json }}`;
     const globalProduct = JSON.parse(globalProductStr.slice(1, -1));