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.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>