core: Fix NPE in DbUserCacheManager

Fix NPE in DbUserCacheManager causing inability to login as
admin@internal if LDAP domain has been add prior to first execution of
engine.

Change-Id: I7703d446841bea47222a066fe0a2b51827f12430
Bug-Url: https://bugzilla.redhat.com/1063276
Signed-off-by: Martin Perina <mperina@redhat.com>