Fix the JS exceptions and incorrect quantity value for checkout event tracking

[eason9487]

Changes proposed in this Pull Request:

Closes #341
Closes #342

• Fix the incompatible data structure that causes JS exceptions when sending checkout event tracking for issue #341.
• Fix the incorrect quantity value when sending checkout event tracking for issue #342.

Checks:

• Does your code follow the WordPress coding standards?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Screenshots:

Detailed test instructions:

1. Install and enable Google Analytics Debugger.
2. Use Woo with version >= 8.1.0.
3. Edit the checkout page to use the Checkout block.
   
4. Open the Console tab in the browser's DevTool.
5. Add items to your cart and go to the Checkout page.
6. Check if there is no longer JS exception in the Console tab.
7. Check if the quantity (~qt\d+~) value in the begin_checkout or checkout_progress event tracking is correct via the Console tab.

Changelog entry

Fix - Avoid JavaScript exceptions when sending checkout event tracking due to incompatible data structure.
Fix - Correct misplaced each product index value as its quantity when sending checkout event tracking.

[jorgemd24]

Thanks @eason9487 for fixing this. I can't see the JS exceptions and the event is now sending the correct qty 👍 . Noticed something odd though— the Google Analytics Debugger displays "Sending event 'xxx' to undefined." Any idea why it's showing as undefined?

[eason9487]

Thanks for quickly helping with the review, @jorgemd24

Noticed something odd though— the Google Analytics Debugger displays "Sending event 'xxx' to undefined." Any idea why it's showing as undefined?

This discussion might be the reason: https://support.google.com/tagmanager/thread/212303562/sending-event-page-view-to-undefined-in-ga-debugger