correctly in the case of FTP_PROXY, because an empty FTP_PROXY has a
specific meaning ("don't use any proxy at all for ftp, even if HTTP_PROXY
is defined"), while an empty HTTP_PROXY has no meaning at all.
PR: bin/85185
Submitted by: Conall O'Brien <conallob=freebsd@maths.tcd.ie>
MFC after: 2 weeks