selftests/exec: Test for empty string on NULL argv

Test for the NULL argv argument producing a single empty string on exec.

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yang Yingliang <yangyingliang@huawei.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20220201011637.2457646-1-keescook@chromium.org