env: ignore unset var
env: ignore unset var

This will substitute the ZSH_VERSION in case it was unset or null. No
more error message will be printed when unset variables are treat as an
error.

Signed-off-by: Christopher Métrailler <1d13d38f8fe373ddca6b2486dbf9672384f71718@nordicsemi.no>

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