thunderbolt: test: use NULL macros

Replace the NULL checks with the more specific and idiomatic NULL macros.
Acked-by: NMika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: NDaniel Latypov <dlatypov@google.com>
Signed-off-by: NRicardo Ribalda <ribalda@chromium.org>
Acked-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>