Make "no members found" message on group members page more general.

'No members were found.' applies both when the group has no members and when
a search within group members returns no members.

Fixes #5935.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9076 cdf35c40-ae34-48e0-9cc9-0c9da1808c22