fix(segmentation): handle bad MC API key

[adekbadek]

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:

Adds handling of invalid Mailchimp API key.
Submitting as a hotfix, because the issue triggers PHP Fatals, which pollute our logs.

How to test the changes in this Pull Request:

1. On release, update the value of the newspack_mailchimp_api_key option to an empty string
2. Run wp eval "\Newspack_Popups_Segmentation::reader_logged_in(111, []);", observe a PHP Fatal is logged
3. Switch to this branch, repeat, observe the Fatal is logged no more

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?

[matticbot]

🎉 This PR is included in version 3.0.1 🎉

The release is available on GitHub release

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