Add test for empty K/M key data handling

(cherry picked from commit f2286a6493c7aab267cf0d9cca680edaf2968707)

https://github.com/krb5/krb5/commit/b371a2c8bbf362df79ee4635a04d50d8b8ba87db
Author: Matt Rogers <mrogers@redhat.com>
Committer: Tom Yu <tlyu@mit.edu>
Commit: b371a2c8bbf362df79ee4635a04d50d8b8ba87db
Branch: krb5-1.14
 src/tests/t_mkey.py |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)