thunderbolt: test: use NULL macros

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

Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>