shell: Fix SHELL_COND_CMD_ARG_REGISTER macro with NULL handler
shell: Fix SHELL_COND_CMD_ARG_REGISTER macro with NULL handler

Macro was failing when handler was NULL and else case was
hit because of attempt to concatenate with handler which was
set to NULL (which is ((void *)0)).

Signed-off-by: Krzysztof Chruscinski <9b6dc988558c51b0405b49b55c8e66862969844b@nordicsemi.no>

galak/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr