fix: handle missing Mailchimp 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:

Fixes an issue with fetching MC data while an API key is not set.

How to test the changes in this Pull Request:

1. On trunk,
2. Remove the newspack_mailchimp_api_key option
3. Run wp eval "\Newspack_Newsletters_Mailchimp_Cached_Data::fetch_segments( '<mc-list-id>' );"* - observe a PHP Fatal is logged
4. Switch to this branch, trigger the command again, observe no error logged

* you can find a valid MC list id by looking up a post_type=newspack_nl_list post's _remote_id post meta

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?

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 20 lines in your changes missing coverage. Please review.

Project coverage is 20.79%. Comparing base (4f206ff) to head (d53837b).
Report is 5 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...ass-newspack-newsletters-mailchimp-cached-data.php	31.03%	20 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk    #1636      +/-   ##
============================================
+ Coverage     20.72%   20.79%   +0.07%     
- Complexity     2378     2386       +8     
============================================
  Files            46       46              
  Lines          8927     8948      +21     
============================================
+ Hits           1850     1861      +11     
- Misses         7077     7087      +10     

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[matticbot]

🎉 This PR is included in version 3.2.0-alpha.2 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 3.2.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀