T5889: Fix migration scripts nat 5-to-6

The current migration drop interface name for NAT where not should

nat {
    source {
        rule 100 {
            outbound-interface {
                name "eth0"
        ...
        }
    }
}

After migration we lost interface:
/home/vyos# /opt/vyatta/etc/config-migrate/migrate/nat/5-to-6 tmp.conf
/home/vyos#
/home/vyos# cat tmp.conf | grep "nat {" -A 10
nat {

source {
    rule 100 {
        outbound-interface {
            interface-name ""
     ...
    }
}

}

This commit fixes it.