Default success conclusion is missing from predicate failure descriptions #1397

When has_status or has_workflow_result is configured without an explicit conclusions list, the predicate correctly defaults to success for evaluation, but the failure description still formats the original (empty) field.

For example:

requires:
  conditions:
    has_status:
      statuses:
        - ci/test

If ci/test concludes with failure, the policy is rejected as expected, but the predicate description is:

One or more statuses has not concluded with : ci/test

The same happens for has_workflow_result:

One or more workflow runs have not concluded with : .github/workflows/ci.yml

In both cases, the condition phrase already says success, so the empty value in the failure message is inconsistent and makes the diagnostic less useful. A user looking at the policy result cannot tell from the failure detail that the implicit requirement was success.

The cause appears to be that Evaluate normalizes the empty configuration into a local success slice for the check itself, but the failure branch still calls pred.Conclusions.joinWithOr() instead of using that normalized value. The fix should be small: use the normalized conclusions when building the failure description, and add regression coverage for the default case in both predicates.