Accept empty password and/or response in auth-pam plugin

Accept empty password and/or response in auth-pam plugin

In the auth-pam plugin correctly parse the static challenge string
even when password or challenge response is empty.

Whether an empty user input is an error is determined by the PAM
conversation function depending on whether the PAM module queries
for it or not.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1533696271-21799-2-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17382.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 7a8109023f4c345fe12f23421c5fa7e88e1ea85b)