fix(mailchimp): prevent merge fields fetch throwing fatal error

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This fetch may fail if the audience does not exist in Mailchimp (Resource not found). This change prevents this error from throwing a fatal error and breaking the donation flow.

How to test the changes in this Pull Request:

1. Make sure you have RAS enabled and Newspack Newsletters is connected Mailchimp
2. Confirm you have define( 'NEWSPACK_LOG_LEVEL', 2 );
3. While on the release branch confirm the error by adding $audience_id = 'foo'; to line 53 of includes/data-events/connectors/class-mailchimp.php
4. Go through the donation flow and confirm the donation order goes through but you get an unhandled error in the UI
5. Check out this branch, apply the same $audience_id = 'foo'; line, and got through the donation flow again
6. Confirm the donation succeeds and the logs shows: [28441][NEWSPACK][Newspack\Data_Events\Connectors\Mailchimp::get_merge_fields]: Error getting merge fields: Resource Not Found

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[dkoo]

Works well! Here's a fix for the same code in Newsletters: Automattic/newspack-newsletters#1376

It's less urgent because it doesn't result in a reader-facing error (although the reader doesn't get subscribed to the list as intended).

[matticbot]

🎉 This PR is included in version 2.11.6 🎉

The release is available on GitHub release

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[matticbot]

🎉 This PR is included in version 2.12.0-alpha.4 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀