Debug-Null-Coalescing-V2

The model receives the prompt (and optional system message). The run uses scorer rubric_json_metrics with the JSON configuration below. Pass/fail and partial credit are determined entirely by that scorer against the model output; no human grading.

Return JSON only with keys diagnosis, fix, tests. Code uses the || operator to fall back to default values; valid falsy inputs such as 0 and empty string are incorrectly replaced by the defaults. Identify the operator bug and fix it.

{
  "metrics": {
    "repro": {
      "checks": [
        {
          "contains": [
            "falsy"
          ]
        },
        {
          "contains": [
            "0"
          ]
        },
        {
          "contains": [
            "empty string"
          ]
        },
        {
          "contains": [
            "default"
          ]
        }
      ]
    },
    "hidden": {
      "checks": [
        {
          "contains": [
            "??"
          ]
        },
        {
          "contains": [
            "nullish coalescing"
          ]
        },
        {
          "contains": [
            "null"
          ]
        },
        {
          "contains": [
            "undefined"
          ]
        }
      ]
    },
    "diagnose": {
      "checks": [
        {
          "contains": [
            "|| operator"
          ]
        },
        {
          "contains": [
            "logical OR"
          ]
        },
        {
          "contains": [
            "falsy values"
          ]
        }
      ]
    }
  }
}