Fix application fee amount handling

[athlonUA]

Summary

This PR ensures that the application_fee_amount value from Stripe's PaymentIntent always defaults to 0 if it is not set. This prevents potential issues with null values and guarantees consistent handling of application fees throughout the payment processing workflow.

Details

• Used the null coalescing operator (?? 0) when accessing $paymentIntent->application_fee_amount in all relevant places.
• This ensures that if Stripe does not provide an application_fee_amount, the system will safely use 0 as the default.
• Improves reliability and prevents errors related to missing or null fee amounts.

Motivation

This change improves the robustness of the payment handling logic by ensuring that all calculations and records involving the application fee are based on a valid integer value, even if Stripe omits the field.

Checklist

• Code follows project standards
• All usages of application_fee_amount are safely defaulted to 0
• Tested to ensure no errors occur when the field is missing

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[athlonUA]

recheck

[Copilot]

Pull Request Overview

This PR ensures that the application fee amount from Stripe's PaymentIntent is always set to a valid integer (0) when not provided, thereby preventing potential null-related errors in fee processing.

• Default the fee amount value to 0 using the null coalescing operator (?? 0).
• Improve the robustness of the payment processing workflow by eliminating null value issues.

[daveearley]

Thanks for this!