tinycrypt/hmac: 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: 143687
Coverity-CID: 143737
Coverity-CID: 143740

This patch is ported from the below patch:
https://gerrit.zephyrproject.org/r/#/c/7418/



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