SNMP: if Macro result is emtpy, use a different macro instead.

[registration_is_lame]

Is it possible to use IFDESCR macro if IFALIAS snmp returns nothing
This is for the discovery interface name.

Currently its item prototype name is"
Interface {IFNAME} - {IFALAIS}

But some devices don't have alias so I'd prefer IFDESCR for them.