This empty folder is what WordPress loads as mu-plugins on the demoing.info
dev server. The parent wp-content/mu-plugins/ folder may contain WP Engine
hosting mu-plugins from UpdraftPlus restores. Those plugins require WPE-only
constants (PWP_NAME, WPE_APIKEY, etc.) and fatal on a non-WPE host.

The redirect lives in wp-config.php (define WPMU_PLUGIN_DIR). UpdraftPlus
does not back up wp-config.php, so when the site is pushed back to WP
Engine, the override is absent and the real WPE mu-plugins load normally.