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 <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Fixes: 378193eff339 ("selftests: kselftest_harness: let PASS / FAIL provide diagnostic")
Tested-by: Sean Christopherson <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

0 files changed, 0 insertions, 0 deletions