net: lwm2m: return NOT_FOUND for unused multi-value resources

When attempting to read an unused multi-value resource such as
"Available Power Sources", lwm2m client return a successful coap packet
w/o any values.  This looks like a timeout to any lwm2m servers which
make sure a valid response is returned.

Let's fix this by returning the correct NOT_FOUND error code instead.

NOTE: To test this I commented out the portion of the lwm2m client
sample which initializes the LWM2M_DEVICE_PWR_SRC_TYPE_BAT_INT and
LWM2M_DEVICE_PWR_SRC_TYPE_USB values.  By default, the sample will
setup dummy values to be returned by the client.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>