dbcheck: fix the err_empty_attribute() check

dbcheck: fix the err_empty_attribute() check

ldb.bytes('') == '' is never True in python3,
we nee ldb.bytes('') == b'' in order to
check that on attribute has an empty value,
that seems to work for python2 and python3.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13843

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Noel Power <npower@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Thu Mar 21 18:15:20 UTC 2019 on sn-devel-144