sanitycheck: Fix bug with -M option
sanitycheck: Fix bug with -M option

If we get a build failure with the -M option we get the following:

TypeError: 'NoneType' object is not iterable

This was due to having the default value of additional_keep in
cleanup_artifacts being None instead of an empty array.

Fixes #29376

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

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