Allow 0 to be stored in activitymeta

Previously, any value where empty( $value ) evaluated to true would result
in the deletion of the row. This change allows for 0 to be stored as a
legitimate value.

This is a stopgap until BP's meta functions are refactored to use WP's core
*_metadata() functions.

Fixes #5083

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7576 cdf35c40-ae34-48e0-9cc9-0c9da1808c22