netlogon: Allow zero-GUID to act the same as NULL in GetDCNameEx2

netlogon: Allow zero-GUID to act the same as NULL in GetDCNameEx2

This matches Windows behaviour and allows rpcclient to work against
Samba without knowing the GUID ahead of time. Errors related to this
don't appear to occur within selftest.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13365

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Fri May  4 09:11:19 CEST 2018 on sn-devel-144