(I am not sure if this better belongs here or node-exporter, feel free to move the issue to the other repo.)

On most ARM devices there are no DMI tables and when using things like u-boot emulating UEFI they are still incomplete. Thus /sys/class/dmi/id/product_serial is not available/empty. Alternatively, however, /sys/firmware/devicetree/base/serial-number is usually available on those devices and also contains a serial number. In case the former does not exists it should fall back to the latter.