[bug] REST API can creates device configs inadvertently

[nemesifier]

This is a follow up of #697 (review).

Create a device with no related configuration.

Open the REST API endpoint to change the device details, eg:
http://localhost:8000/api/v1/controller/device/{pk}/

Now, hit the PUT button without changing anything.

Expected result:

• device is saved but config is not created because no data was added

Actual result:

• config is created empty

If I reset the repository state to before #697, an exception is raised.
After merging #697 the exception is not raised anymore and the config is created.
Ideally we shouldn't create/change the config unless anything in the config object is added/changed.