Make it possible to distinguish between no updates available and no updates supported

[raman325]

Today, if you query the firmware update service, you will only see updates if there are firmware updates available for the device in question. When you don't see any updates, however, it's hard to know whether that means that the device IS supported by the update service but has no updates available or the device ISN'T supported by the update service and therefore has no updates available.

One suggestion @AlCalzone had on Discord was to return a null instead of an empty list to indicate that the device is not supported by the service. Not sure if this needs to go into the service itself or into the caller, but starting here for tracking purposes

[AlCalzone]

One thing we may have to consider: Some devices are gated by firmware version, e.g. the recently Aeotec devices. If you are on their latest version, no file will match, so without further checks (i.e. does a file exist for any firmware version?), the service response would be not supported. So I guess we'd also need to check if there is an update available to the current version.