In `bp_xprofile_updated_profile_activity()`, check that `$new_value['visibility']` isn't empty before comparing against it.

This prevents debug notices in custom profile save routines where field visibility is not integrated.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8814 cdf35c40-ae34-48e0-9cc9-0c9da1808c22