fix: ensure localized segments object is an object, not array

[dkoo]

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:

Casts localized segments data as an object to ensure that it's of the expected type when read by the segmentation API. Avoids potential type errors in JS.

How to test the changes in this Pull Request:

1. On release, disable or delete all segments in Newspack > Campaigns > Segments.
2. On the front-end, in the JS console type newspack_popups_view.segments and observe that it's an empty array:

3. Check out this branch, refresh, and confirm that it's now an object:

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?

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[matticbot]

🎉 This PR is included in version 3.1.2 🎉

The release is available on GitHub release

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