Update subscription.rb (#1168)

Fix: Handle empty payments array in Stripe subscription swap

**Problem:**

A `NoMethodError: undefined method 'payment' for nil:NilClass` was occurring in `Pay::Stripe::Subscription#swap` when a user changed their subscription plan, and the change resulted in a credit or a $0 invoice (e.g., due to proration).

**Root Cause:**

When a Stripe subscription update results in a credit or a $0 invoice, the `latest_invoice` object returned by the Stripe API has an empty `payments` array. This is because no actual payment is processed, and therefore no `payment_intent` is generated for such invoices. The existing code was attempting to access `payments.first.payment.payment_intent` without checking if `payments.first` was `nil`.

**Solution:**

This commit updates the `swap` method to use safe navigation operators (`&.`) when accessing `latest_invoice.payments.first.payment.payment_intent`.

```ruby
# Before
# if (payment_intent_id = @api_record.latest_invoice.payments.first.payment.payment_intent)

# After
if (payment_intent_id = @api_record.latest_invoice.payments.first&.payment&.payment_intent)

Author: botsarenthuman

app/models/pay/stripe/subscription.rb

@@ -297,7 +297,7 @@ def swap(plan, **options)
         )
 
         # Validate that swap was successful and handle SCA if needed
-        if (payment_intent_id = @api_record.latest_invoice.payments.first.payment.payment_intent)
+        if (payment_intent_id = @api_record.latest_invoice.payments.first&.payment&.payment_intent)
           Pay::Payment.from_id(payment_intent_id).validate
         end