fix(reader-registration-block): fix initial newsletter checkbox state

[adekbadek]

All Submissions:

• [x] Have you followed the Newspack Contributing guideline?
• [x] Does your code follow the WordPress' coding standards and VIP Go coding standards?
• [x] 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 initial state of the newsletter checkboxes in the Reader Registration block (#2682)

How to test the changes in this Pull Request:

1. On trunk, insert the Reader Registration block on a page and select at least two lists to be displayed in the sidebar block settings
2. Check one of the lists in the block, to make it selected by default
3. Publish the page and visit the frontend – observe both checkboxes are pre-selected
4. Switch to this branch, reload, observe only the list selected in the editor is pre-selected

Other information:

• [x] 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?
• [x] Have you successfully ran tests with your changes locally?

[matticbot]

:tada: This PR is included in version 5.4.0-alpha.6 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket:

[matticbot]

:tada: This PR is included in version 5.5.0-alpha.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: