When updating meta cache for a group of items, make sure empty values are cached

When updating the meta cache for, say, a group of activity items, it's possible
that some activity items won't have any metadata associated with them. In these
cases, we follow WP's update_meta_cache() by caching an empty array for that
item, so as to avoid cache misses.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8011 cdf35c40-ae34-48e0-9cc9-0c9da1808c22