Steps to reproduce:
If Database table has a reference for a keytab file at /var/lib/ambari-server/data/cache/ and if file is not present then Ambari would copy empty keytab files to all the hosts.
1. Install and Kerberize the cluster.
2. remove keytab file/s at /var/lib/ambari-server/data/cache/
3. Now re-generate missing keytab files operation.
4. You can find empty keytab file in cluster hosts.
Ambari should check keytab file exist or not while deciding what to re-generate.
This BUG can cause issue during the Upgrade as it runs re-generate missing keytab operation.