Fixes #6951

Changes proposed in this Pull Request

In the event that no payment method ID is found when setting the payment method title for emails, this will provide a fallback, setting the payment method title to $this->title or WooPayments. This will avoid any errors that might occur if the payment method ID is empty.

Testing instructions

I haven't been able to reproduce the fatal error that's mentioned in #6951, even when setting $payment_method_id to an empty string. But this can be tested manually:

Normal Order Flow

1. Set $payment_method_id to an empty string here.
2. Enable UPE checkout (legacy) using WCPay Dev Tools
3. Add a product to the cart and proceed to checkout
4. Switch to EUR currency and check that Popular Payments Methods is the heading for payment methods
5. Complete checkout using credit card
6. Check that the payment method title in the emails sent, such as the New Order and Processing Order emails, is WooPayments.
7. Revert the change in step 1 above.
8. Repeat the process and ensure the correct payment method title appears in the emails.
9. Check fatal error logs to be sure no errors are occurring

Pre-orders

The fatal error that surfaced this issue occurred during a pre-order flow. Let's test WooCommerce Pre-Orders as well.

1. Install Pre-Orders
2. Enable pre-orders on a product
3. Set a release date in the future
4. Set When to Charge to Upfront
5. Repeat the steps in the Normal Order Flow above, this time purchasing the pre-order product.
6. After each order, go to WooCommerce > Pre-orders and complete the order using the checkbox and dropdown.
7. Check the emails sent for the pre-order completion for the appropriate payment method title
8. Check fatal error logs to be sure no errors are occurring

• Run npm run changelog to add a changelog file, choose patch to leave it empty if the change is not significant. You can add multiple changelog files in one PR by running this command a few times.
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Post merge

• Link to testing instructions from release testing doc following these instructions : Add link here / 'QA Testing Not Applicable'
• Add or update critical flows and testing instructions for critical flows, if applicable.
• Add what's changed (description, screenshot, demo videos etc.) to the release announcement post, if applicable.