core: Fixing NPE in case a directory was not found for user.

This can happen if the domain was removed by engine-manage-domains
(the users for the directory are not removed)

Change-Id: Ief4761b9234f3839a1dba1eefbf871ae2bae3c69
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>