Fix duplicate InitiateCheckout when using checkout block.

[rawdreeg]

Changes proposed in this Pull Request:

When using checkout WC Block, initiateCheckout event is triggered on the order received page. This causes an incomplete event to be sent to Facebook causing a warning. This PR adds an additional check to confirm the cart is not empty.

Closes #2348 .

• [ x] Do the changed files pass phpcs checks? Please remove phpcs:ignore comments in changed files and fix any issues, or delete if not practical.

Detailed test instructions:

1. Ensure checkout form uses Checkout WC Block
2. Add product to cart and navigate to checkout. Using the Facebook Pixel Helper chrome extension, confirm the initiateCheckout event is properly triggered.
3. Confirm the initiateCheckout event is not triggered a second time on the order received page

Additional details:

I came across this issue while investigating #2075.

Changelog entry

Fix - duplicate InitiateCheckout when using checkout block.

[message-dimke]

Any tip on how to reproduce the issue?

This is on develop branch, no InitiateCheckout at Order Received page.

[rawdreeg]

Any tip on how to reproduce the issue?

Hey @message-dimke, this seems to occur when the WC Checkout block is used. I made a video reproducing the issue:

Screen.Recording.Duplicate.InitCheckout.mov

[message-dimke]

Seems good, thanks @rawdreeg !