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 <ramakrishna.pallala@intel.com>