Don't skip filter in BP_XProfile_Field::get_children() when no children are found

Even null results should go through the 'bp_xprofile_field_get_children'
filter, so that developers can add children dynamically.

Fixes #5692

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8478 cdf35c40-ae34-48e0-9cc9-0c9da1808c22