Add default values to `bp_core_get_userid_from_nicename()` and `bp_core_get_userid()` so any usages without passed parameters do not cause debug notices (that's why those `empty()` checks are there, obvs.)

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8622 cdf35c40-ae34-48e0-9cc9-0c9da1808c22