Ensure that a last_activity value is set for each user in BP_User_Query::populate_extras()

This helps to avoid PHP notices when directly accessing this value for a user
who has not yet logged activity.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8054 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
POT, generated from r8054

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8055 cdf35c40-ae34-48e0-9cc9-0c9da1808c22