net: lwm2m: fix error code in read and write handlers
net: lwm2m: fix error code in read and write handlers

When a data pointer or data length is not set, the read and write
handlers should return ENOENT to generate the correct LwM2M error
code (COAP_RESPONSE_CODE_NOT_FOUND).

Signed-off-by: Michael Scott <17b9e1c64588c7fa6419b4d29dc1f4426279ba01@opensourcefoundries.com>

zephyriot/zephyr,explora26/zephyr,mbolivar/zephyr,mbolivar/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,zephyriot/zephyr,finikorg/zephyr,kraj/zephyr,explora26/zephyr,punitvara/zephyr,zephyriot/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,punitvara/zephyr,zephyriot/zephyr,galak/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,punitvara/zephyr,nashif/zephyr,punitvara/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,mbolivar/zephyr,zephyriot/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,ldts/zephyr,Vudentz/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr