s4:dlz make b9_has_soa check dc=@ node

s4:dlz make b9_has_soa check dc=@ node

the zone node does not hold the dnsRecord values, so for the zone level
the node dc=@,dc=zonename has to be queried

regression introduced with 28e2a518ff32, BUG: https://bugzilla.samba.org/show_bug.cgi?id=13466

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13841

Signed-off-by: Michael Saxl <mike@mwsys.mine.bz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 526c6d0be53d97beb38f82a3619d8710fefb4091)