Fix segmentation fault in oned service when configured to cache mode.

[alonbo2001]

Issue #6302
Description
After setting federation mode to be "CACHE" the XML/RPC server crash on segmentation fault.

To Reproduce

• Configure "CACHE" API server - in /etc/one/oned.conf set federation mode to "CACHE" and set DB and MASTER_ONED.
• sudo systemctl restart opennebula
• send "one.zone.raftstatus" request to server.

Detailes

• Hypervisor: KVM
• version: 6.4.0

Program crashed when trying to call hm->trigger_send_event in Request.cc (hm is null).

Before :

if (!event.empty())
{
    hm->trigger_send_event(event);
}

After:

if (!nd.is_cache())
{
    if (!event.empty())
    {
        hm->trigger_send_event(event);
    }
}

When nebula is cache HookManager isn't initialize, therfore hm equals to null.

[rsmontero]

Thanks! Merged as per d74ace5