device: use empty string as drvname when devicetree label is absent

As we move to making the `label` property optional, we need to account
for widespread use of `dev->name` in contexts where the pointer is not
validated before it is dereferenced.  Use an empty string instead of a
null pointer when no label is provided.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>