WP Super Cache: Fix $wp_cache_request_uri checks so WP CLI works properly

As reported [here](https://wordpress.org/support/topic/wp-super-cache-and-scheduled-posts/#post-17881391). If $wp_cache_request_uri is null or blank as it will be when the WP CLI is used then actions aren't hooked, which means that scheduled posts will not clear the cache if the cron system is triggered by the CLI.