krb5_get_init_creds_password with empty password and no prompter

[Stef Walter]

When krb5_get_init_creds_password() is called with an empty password
(non-null, empty string) and NULL prompter, then it returns
KRB5_PREAUTH_FAILED

This prevents krb5_get_init_creds_password() use of accounts with an
empty password (although that's a corner case). When one provides a
prompter that returns nothing, suddenly things work as expected:

https://bugzilla.redhat.com/show_bug.cgi?id=960001#c1

I've tried to fix this in preauth2.c get_as_key() without success. Any
tips on a good way to tackle/patch this?

Thanks in advance,

Stef