net: lib: lwm2m: Cancel firmware DOWNLOADED state with NULL byte

Allow to cancel a firmware update in DOWNLOADED state by writing a
NULL byte as described in the Firmware Update object version 1.1
(urn:oma:lwm2m:oma:5:1.1). Keep object version 1.0 mechanism with
an `empty string`.

Signed-off-by: Marc Lasch <marc.lasch@husqvarnagroup.com>