Use an empty challenge for the password question

If a question's challenge is NULL, it is unnecessarily difficult for a
responder callback to detect whether it was asked.  So it's better to
use an empty challenge when there is no challenge data to communicate.
Do this for the "password" question.

(cherry picked from commit 70f2d9a093c71624269b2317c62ad0993126bc40)

https://github.com/krb5/krb5/commit/e24793e6b55d41dab55bbb53d6375177195023a2
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: e24793e6b55d41dab55bbb53d6375177195023a2
Branch: krb5-1.11
 src/lib/krb5/krb/gic_pwd.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)