if key_ in dservices null and zabbix_server trying search in DB row with discvered service, it execute SQL:
select dserviceid,dhostid,status,lastup,lastdown,value from dservices where dcheckid=48 and type=12 and key_='' and ip='10.100.187.198' and port=0;

but for Oracle key_='' not key_=null

for Oracle need query:
select dserviceid,dhostid,status,lastup,lastdown,value from dservices where dcheckid=48 and type=12 and key_ is null and ip='10.100.187.198' and port=0;

I think in sources still encountered similar problems.