Deprecated notice: `Using null as an array offset is deprecated, use an empty string instead`

Our sites live on VIP environments, and the logs are cluttered with zillions of deprecation notices about a null array offset on `wp-memcached/includes/stats.php:87`. It makes it hard to see anything else in the logs. It would be great if someone could address this in an upcoming release of `wp-memcached`!

Example:

```
PHP message: Deprecated: Using null as an array offset is deprecated, use an empty string instead in /var/www/wp-content/mu-plugins/drop-ins/wp-memcached/includes/stats.php on line 87 [...-develop.go-vip.net] [wp-content/mu-plugins/drop-ins/wp-memcached/includes/wp-object-cache.php:1098 Automattic\Memcached\Stats->group_ops_stats(), wp-content/mu-plugins/drop-ins/wp-memcached/includes/wp-object-cache.php:175 WP_Object_Cache->group_ops_stats(), wp-content/mu-plugins/drop-ins/wp-memcached/object-cache.php:57 WP_Object_Cache->add(), wp-content/mu-plugins/cron/cron-control/includes/class-lock.php:96 wp_cache_add(), wp-content/mu-plugins/cron/cron-control/includes/class-events.php:46 Automattic\WP\Cron_Control\Lock::prime_lock(), wp-content/mu-plugins/cron/cron-control/includes/class-singleton.php:32 Automattic\WP\Cron_Control\Events->class_init(), wp-content/mu-plugins/cron/cron-control/includes/class-main.php:87 Automattic\WP\Cron_Control\Singleton::instance(), wp-content/mu-plugins/cron/cron-control/includes/class-main.php:24 Automattic\WP\Cron_Control\Main->load_plugin_classes(), wp-content/mu-plugins/cron/cron-control/includes/class-singleton.php:32 Automattic\WP\Cron_Control\Main->class_init(), wp-content/mu-plugins/cron/cron-control/cron-control.php:23 Automattic\WP\Cron_Control\Singleton::instance(), wp-content/mu-plugins/001-cron.php:126 require_once('wp-content/mu-plugins/cron/cron-control/cron-control.php'), wp-settings.php:500 include_once('wp-content/mu-plugins/001-cron.php'), /usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php:1374 require('wp-settings.php'), /usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php:1293 WP_CLI\Runner->load_wordpress(), /usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Bootstrap/LaunchRunner.php:28 WP_CLI\Runner->start(), /usr/local/bin/wp/vendor/wp-cli/wp-cli/php/bootstrap.php:84 WP_CLI\Bootstrap\LaunchRunner->process(), /usr/local/bin/wp/vendor/wp-cli/wp-cli/php/wp-cli.php:35 WP_CLI\bootstrap(), /var/wpvip/fpm-cron-runner.php:80 require_once('/usr/local/bin/wp/vendor/wp-cli/wp-cli/php/wp-cli.php')]
```

Start by reading wp-memcached/includes/stats.php around line 87, then trace the stats call from wp-memcached/includes/wp-object-cache.php as shown in the log. Reproduce or inspect the affected stats path and confirm that the deprecation notice no longer appears when it runs.

Written by the indexing model from the issue text.