Only get 'search_terms' out of $_REQUEST['s'] in bp_group_has_members() if no 'search_terms' param has been passed to the function

This allows secondary, search-free loops to be run on a page, by passing an
empty string to the 'search_terms' param. See #5318 for a similar strategy as
applied to members widgets.

Fixes #5746

Props WCUADD

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8574 cdf35c40-ae34-48e0-9cc9-0c9da1808c22