Add test for empty K/M key data handling

(back ported from commit f2286a6493c7aab267cf0d9cca680edaf2968707)

https://github.com/krb5/krb5/commit/75d61d3875e61679610a02f2b1b4bbac9af234cf
Author: Matt Rogers <mrogers@redhat.com>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 75d61d3875e61679610a02f2b1b4bbac9af234cf
Branch: krb5-1.13
 src/tests/t_mkey.py |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)