fix(payments-next): Fix update button for Link as saved method

[david1alvarez]

Because:

• When the user had Link as their saved payment method, the Manage Payment Methods page would show the option to update/save their payment method even when no changes had been made

This commit:

• Fixes the check to require a payment method ID. This results in the Payment Element correctly showing the "Set as default" button when a change is made, and not showing the button when no change is made.

Closes #PAY-3422

Checklist

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Screenshots (Optional)

The PaymentMethod element no longer shows the button to set as default when Link is the default

Changing the payment method, and then changing back to Link still shows the button

Other information (Optional)

Any other information that is important to this pull request.

[elizabeth-ilina]

I was able to reproduce the issue in this situation:

link-default-pm-bug.mov

What appears to be causing it is that in this situation, event.value.payment_method.id !== defaultPaymentMethodId (even though they should because the user only has 1 payment method saved). When a user clicks the button to save their current Link payment method as default, the event.value.payment_method.id and defaultPaymentMethodId switch values, and so they don’t match again. As the user clicks the button again, these values keep switching back and forth, and the "Set as default payment method" button is being displayed:

link-default-pm-switching-ids.mov

[david1alvarez]

The new behavior can be seen here. The non-optimal UX highlighted here is being tracked in PAY-3459, as this appears to be the component working as intended by Stripe.
https://github.com/user-attachments/assets/13e83e7b-e071-4b30-95a4-f2cff740ddee