selftests: ktap_helpers: Fix uninitialized variable

selftests: ktap_helpers: Fix uninitialized variable

__ktap_test() may be called without the optional third argument which is
an issue for scripts using `set -u` to detect uninitialized variables
and potential bugs.

Fix this optional "directive" argument by either using the third
argument or an empty string.

This is required for the next commit to properly test script execution
control.

Cc: Kees Cook <kees@kernel.org>
Cc: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Fixes: 14571ab1ad21 ("kselftest: Add new test for detecting unprobed Devicetree devices")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-7-mic@digikod.net
Signed-off-by: Kees Cook <kees@kernel.org>