pppd: Fix blank password usage

If a password has been provided as "", CHAP authentication wouldn't
happen. A user providing a username/password, even if blank, should be
expecting authentication to occur with those set.

Added a check for explicit_passwd property, set on finding the password
argument, to allow CHAP authentication with a blank password.

Signed-off-by: Simon Tate <simon.tate@bt.com>