Show 4 latest posts when site.related_posts is nil

`site.related_posts` should return an array of latest posts when no related ones are found due to `lsi` being disabled (GitHub Page's doesn't support it).

This works around that bug by showing the 4 latest posts to populate the "You May Also Enjoy" module when enabled with `related: true`.

Fixes #554
Related: #978 #1022

CHANGELOG.md

@@ -11,6 +11,7 @@
 - Improve Chinese `date_label` and `minute_read` translations in `ui-text.yml`. [#1205](https://github.com/mmistakes/minimal-mistakes/pull/1205) [#1211](https://github.com/mmistakes/minimal-mistakes/pull/1211)
 - Add note to Quick-Start Guide about GitHub Pages hosting alternatives that allow 3rd party gem themes and Jekyll plugins.
 - Add note to configuration documentation about Cloudflare minification as an alternative to `layout: compress`. [#1217](https://github.com/mmistakes/minimal-mistakes/pull/1217)
+- Show 4 latest posts in "You May Also Enjoy" module when `related: true` and no related posts are found due to `lsi` ([latent semantic indexing](https://en.wikipedia.org/wiki/Latent_semantic_analysis#Latent_semantic_indexing)) being disabled on GitHub Pages. [#554](https://github.com/mmistakes/minimal-mistakes/issues/554)
 
 ### Bug Fixes