Hotfix/handle empty taxonomies - NPPM-2460

[leogermani]

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:

Taxonomy terms are kept in sync on content distribution. However, when all terms are removed in the original post, the taxonomy is completely ignored, and the terms are never deleted from distributed posts

How to test the changes in this Pull Request:

1. Create a post, add some tags, and distribute it
2. Check the distributed post has the correct tags in the target site
3. Edit the origin post and remove one tag - but leaving others. save it
4. Confirm the tag is properly removed from the distributed post
5. Back to the origin post, remove all tags and save
6. Confirm all tags are removed from the distributed post.

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?

[miguelpeixe]

Tests well! Don't forget to rebase to release before merging.

[matticbot]

🎉 This PR is included in version 2.18.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀