Use default product backup image

1. The image from a product variant isn't garunteed to exist. In the rest
    of the codebase we always use the first product image (product.image[0])
    in case the variant image doesn't exist.
2. Lets do that here too. We can also prefer the thumbnail image because
    its smaller and we don't need a large image. If that doesn't exist
    either lets fall back to the default product image.
3. The product image isn't garunteed to exist either, but if it doesn't
    that means there is no image associated with the product.

frontend/templates/troubleshooting/Resource.tsx

@@ -60,7 +60,11 @@ export function ProductResource({ product }: { product: Product }) {
       <Resource
          href={productUrl}
          title={product.title}
-         imageUrl={selectedVariant.image?.url}
+         imageUrl={
+            selectedVariant.image?.url ||
+            product.images[0]?.thumbnailUrl ||
+            product.images[0]?.url
+         }
          spacing="4px"
          showBuyButton={isForSale}
          openInNewTab={true}