When using Zabbix proxy inconsistent autoregistration behaviour for hosts w/o metadata on MySQL vs Oracle

This issue with autoregistration when using proxy is similar to what was happening ZBX-18364. With Oracle DB already discovered hosts are updated every time when autoregistration is processed even when nothing has changed.

It is expected to process autoregistration only when proxy ID or host metadata has changed.

In src/libs/zbxdbhigh/db.c:process_autoreg_hosts()

if (current_proxy_hostid != proxy_hostid || SUCCEED == DBis_null(row[3]) ||
        0 != strcmp(autoreg_host->host_metadata, row[3]))
{
        break;
}

It checks DBis_null(row[3]) where row[3] is the value of host_metadata field. host_metadata can be NULL or '\0' on MySQL but '\0' for Oracle in both cases - when no data and empty string.