selftests: kselftest_harness: fix Clang warning about zero-length format

Apparently it's more legal to pass the format as NULL, than
it is to use an empty string. Clang complains about empty
formats:

./../kselftest_harness.h:1207:30: warning: format string is empty
[-Wformat-zero-length]
 1207 |            diagnostic ? "%s" : "", diagnostic);
      |                                 ^~
1 warning generated.
Reported-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/all/20240409224256.1581292-1-seanjc@google.com
Fixes: 378193ef ("selftests: kselftest_harness: let PASS / FAIL provide diagnostic")
Tested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20240416151048.1682352-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/kselftest.h

@@ -288,15 +288,17 @@ void ksft_test_result_code(int exit_code, const char *test_name,
 	}
 
 	/* Docs seem to call for double space if directive is absent */
-	if (!directive[0] && msg[0])
+	if (!directive[0] && msg)
 		directive = " #  ";
 
-	va_start(args, msg);
 	printf("%s %u %s%s", tap_code, ksft_test_num(), test_name, directive);
 	errno = saved_errno;
+	if (msg) {
+		va_start(args, msg);
 		vprintf(msg, args);
-	printf("\n");
 		va_end(args);
+	}
+	printf("\n");
 }
 
 static inline int ksft_exit_pass(void)

tools/testing/selftests/kselftest_harness.h

@@ -1202,7 +1202,7 @@ void __run_test(struct __fixture_metadata *f,
 		diagnostic = "unknown";
 
 	ksft_test_result_code(t->exit_code, test_name,
-			      diagnostic ? "%s" : "", diagnostic);
+			      diagnostic ? "%s" : NULL, diagnostic);
 }
 
 static int test_harness_run(int argc, char **argv)