What

This PR adds a graphql_debug() message when null is returned from a Page.seo on the Posts archive

Why

Since WPGraphQL doesn't have an archive type, it treats the page_for_posts WP_Post object as a ContentType instead of content node. ( See the discussion on #2486 ).

As that's not readily obvious, a debug message directing them to use contentType or nodeByUri will let them know that it's returning null for a reason.
 
Closes #111

How

If the URI is null and the a Post model is for the Posts Page, logs a message to extensions.debug

Testing Instructions

Additional Info

Checklist:

• My code is tested to the best of my abilities.
• My code follows the WordPress Coding Standards.
• My code has proper inline documentation.
• I have added unit tests to verify the code works as intended.
• The changes in this PR have been noted in CHANGELOG.md