tinycrypt/sha256: Array compared to NULL has no effect
tinycrypt/sha256: Array compared to NULL has no effect

This commit fixes the issue reported by Coverity: an array compared
against NULL is always false.

Coverity-CID: 143715
Coverity-CID: 143730

THis patch is ported from below patch:
https://gerrit.zephyrproject.org/r/#/c/7419/

Jira : ZEP-2468
Signed-off-by: Ramakrishna Pallala <8cb5df094165bcc31e7d772ccd8d87d8c4d8f761@intel.com>

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