__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 <redacted>
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 (local)
---

Also sent as a standalone patch:
https://lore.kernel.org/r/20241127160342.31472-1-mic@digikod.net (local)
---
 tools/testing/selftests/kselftest/ktap_helpers.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/kselftest/ktap_helpers.sh b/tools/testing/selftests/kselftest/ktap_helpers.sh
index 79a125eb24c2..14e7f3ec3f84 100644
--- a/tools/testing/selftests/kselftest/ktap_helpers.sh
+++ b/tools/testing/selftests/kselftest/ktap_helpers.sh

@@ -40,7 +40,7 @@ ktap_skip_all() {
 __ktap_test() {
 	result="$1"
 	description="$2"
-	directive="$3" # optional
+	directive="${3:-}" # optional
 
 	local directive_str=
 	[ ! -z "$directive" ] && directive_str="# $directive"

-- 
2.47.1