global $post is not set in REST API context

[joemaller]

This is blowing up the editor:

https://github.com/ideasonpurpose/wp-related-posts/blob/0328b076468e87a836e18c8dd1dfb671ebd28402/src/RelatedPosts.php#L330-L338

Two things need to happen:

• [ ] 1. The editor block needs to send the post ID along with weights and post_types
• [x] 2. The library needs to fall back to the most recently published post if nothing is provided.

[joemaller]

improved fallbacks in a2086aa