net: shell: Fix build when NET_DEBUG_APP defined, but client/server not
net: shell: Fix build when NET_DEBUG_APP defined, but client/server not

If CONFIG_NET_DEBUG_APP was defined, but neither NET_APP_SERVER nor
NET_APP_CLIENT, build failed due to net_app_cb() haven't beeen
defined. So, define it to empty in this case.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

mbolivar/zephyr,punitvara/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,explora26/zephyr,zephyriot/zephyr,kraj/zephyr,finikorg/zephyr,explora26/zephyr,Vudentz/zephyr,zephyriot/zephyr,kraj/zephyr,aceofall/zephyr-iotos,finikorg/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,mbolivar/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3220,zephyriot/zephyr,nashif/zephyr,mbolivar/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3220,ldts/zephyr,galak/zephyr,runchip/zephyr-cc3220,nashif/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,galak/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,kraj/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,Vudentz/zephyr,ldts/zephyr,zephyriot/zephyr,nashif/zephyr,punitvara/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,mbolivar/zephyr,nashif/zephyr,fbsder/zephyr,fbsder/zephyr,kraj/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,explora26/zephyr,punitvara/zephyr,finikorg/zephyr,fbsder/zephyr,galak/zephyr,runchip/zephyr-cc3220,finikorg/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyriot/zephyr