fix: prevent prompt expiration with invalid expiration date

[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 a regression introduced in https://github.com/Automattic/newspack-popups/pull/1305. For some reason, some posts did get expiration_date with empty value assigned. Unfortunately, there's no way to fix these (re-publish), since we can't know which prompts were supposed to be published or not in the first place.

How to test the changes in this Pull Request:

1. On release, assign empty value to a published prompt expiration_date meta
2. Run wp cron event run newspack_popups_check_expiry, observe the prompt has been reverted to draft
3. Switch to this branch, repeat, observe the prompt with invalid expiration_date is not reverted to draft

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you written new tests for your changes, as applicable?
• [x] Have you successfully ran tests with your changes locally?

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[matticbot]

:tada: This PR is included in version 2.33.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: