Add the backup to make it backward compatible

Author: Kyon147

src/resources/views/layouts/default.blade.php

@@ -18,8 +18,8 @@
         </div>
 
         @if(\Osiset\ShopifyApp\Util::getShopifyConfig('appbridge_enabled') && \Osiset\ShopifyApp\Util::useNativeAppBridge())
-            <script src="{{config('shopify-app.appbridge_cdn_url')}}/@shopify/app-bridge{{ \Osiset\ShopifyApp\Util::getShopifyConfig('appbridge_version') ? '@'.config('shopify-app.appbridge_version') : '' }}"></script>
-            <script src="{{config('shopify-app.appbridge_cdn_url')}}/@shopify/app-bridge-utils{{ \Osiset\ShopifyApp\Util::getShopifyConfig('appbridge_version') ? '@'.config('shopify-app.appbridge_version') : '' }}"></script>
+            <script src="{{config('shopify-app.appbridge_cdn_url') ?? 'https://unpkg.com'}}/@shopify/app-bridge{{ \Osiset\ShopifyApp\Util::getShopifyConfig('appbridge_version') ? '@'.config('shopify-app.appbridge_version') : '' }}"></script>
+            <script src="{{config('shopify-app.appbridge_cdn_url') ?? 'https://unpkg.com'}}/@shopify/app-bridge-utils{{ \Osiset\ShopifyApp\Util::getShopifyConfig('appbridge_version') ? '@'.config('shopify-app.appbridge_version') : '' }}"></script>
             <script
                 @if(\Osiset\ShopifyApp\Util::getShopifyConfig('turbo_enabled'))
                     data-turbolinks-eval="false"