fix(diskutil): Fix ReferenceError when DeviceNode is undefined
fix(diskutil): Fix ReferenceError when DeviceNode is undefined

This fixes a ReferenceError that could occur when a device is
being unmounted & unplugged from the host, where `DeviceNode`
would be empty.

Change-Type: patch

resin-io-modules/drivelist,resin-io/drivelist,resin-io-modules/drivelist,resin-io-modules/drivelist,resin-io-modules/drivelist,resin-io/drivelist