must define isEmpty

When using APC for the Drupal cache, I get the following error:

[Thu Dec 24 16:16:05 2009] [error] [client 68.116.70.50] PHP Fatal error:  Class DrupalAPCCache contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (DrupalCacheInterface::isEmpty) in /var/www/html/drupal-7.x-dev-apc/sites/default/modules/apc/drupal_apc_cache.inc on line 249

The attached patch implements an isEmpty() function that always returns FALSE, mimicking the memcache module, and as recommended in this comment.