fix(payments-next): resolve linting error

Because:

* a variable could be undefined, and one of its attributes is accessed unsafely

This commit:

* safely access the attribute

Closes #N/A

Author: david1alvarez

libs/payments/cart/src/lib/cart.service.ts

@@ -789,7 +789,7 @@ export class CartService {
         metricsOptedOut,
         latestInvoicePreview,
         paymentInfo,
-        hasActiveSubscriptions: !!subscriptions.length,
+        hasActiveSubscriptions: !!subscriptions?.length,
       };
     }