fix(settings): get_setting method

[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:

Fixes a logical error in the Newspack_Popups_Settings::get_setting method. If getting an invalid setting key or a setting key that doesn't have a saved option, it would always return true instead of the setting's default value. This PR validates the key and also returns the default value if a value hasn't been saved before.

How to test the changes in this Pull Request:

1. On a fresh site, install Campaigns from master.
2. Run wp shell, then Newspack_Popups_Settings::get_setting( 'newspack_popups_mc_donor_merge_field' ). Observe that it returns true.
3. Check out this branch and repeat step 2 in a new wp shell session. Confirm it now returns DONAT.
4. Update the settings in Newspack > Campaigns > Settings and repeat with various keys (see here for keys to check).

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 2.26.1 🎉

The release is available on GitHub release

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