kunit: use NULL macros

Replace the NULL checks with the more specific and idiomatic NULL macros.

Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>