Skip instead of throwing error when product id is not found for legacy Ajax add-to-cart

[triple0t]

Changes proposed in this Pull Request:

Closes #451

This PR skips the rest of the checks and consoles the error message instead of throwing the error, which might interfere with other scripts or processing.

The main aim of this PR is not to change the functionality, but rather to ensure that errors thrown here do not interfere with other scripts that might depend on the same event listener.

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:

Additional details:

Changelog entry

Tweak - Don't throw errors when the product ID is not found for legacy Ajax add-to-cart

[eason9487]

Thanks for the work! LGTM.

Left two questions regarding the need for using optional chaining operators, but they are not blockers.