Allow output of `bp_get_send_public_message_link()` to still be filtered even when empty. Also use `add_query_arg()` and break `$url` building up into easier to read lines.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8729 cdf35c40-ae34-48e0-9cc9-0c9da1808c22